The building from which a gunman fired at former President Trump on Saturday was — at least in hindsight — an obvious security risk. Its rooftop offered an ideal sniper’s perch, with a close, elevated and unobstructed view of Mr. Trump.

But when the Secret Service drew up plans for Saturday’s rally, it left that building outside its security perimeter. Instead, local law enforcement officials in Butler, Pa., were given responsibility for that building, and no police officers were stationed on the roof itself.

The building, used as a warehouse by equipment manufacturer AGR International, has become a focal point of myriad investigations into the shooting that nearly felled a former American president, one that Homeland Security Secretary Alejandro N. Mayorkas on Monday called a security failure.

The first question is why the building, about 450 feet from the stage, was left out of the perimeter. A Secret Service advance team visited the site and made the determination, and a supervisor would have had to approve it. The agency so far has not said who that was.

Video

That is just one of many unanswered questions. It is also unclear how the gunman got on the roof. People at the rally reported a suspicious person to local law enforcement. Quickly thereafter, rally visitors pointed out a man on the warehouse and the Secret Service shot and killed him after he began his assault.

The Secret Service has not said whether local law enforcement officers made service agents aware of a suspect or whether those officers were up to the task of dealing with the situation.

But the central question is whether the Secret Service failed at its most basic mission: keeping America’s leaders, including a former president, safe.

The security lapse is now the center of overlapping investigations, run by members of Congress and the Secret Service itself. On Monday, the leaders of the Senate’s Homeland Security Committee announced their own investigation into Saturday night’s shooting. Senators Gary Peters, Democrat of Michigan, and Rand Paul, Republican of Kentucky, told the Secret Service in a letter that they wanted to know “how the suspect was able to get this close to a Secret Service protectee.”

“An incident like this cannot happen,” Mr. Mayorkas told CNN, adding, “When I say something like this cannot happen, we are speaking of a failure.”

Former federal law enforcement officials have said the Secret Service should have ensured the building was secured before the rally took place. The agency often relies on local law enforcement agencies for security at events.

One witness to Saturday’s shooting said that he had been allowed to walk into the same area as the warehouse without a security check.

Nathan Steadman said that he and his daughter stood under a nearby tree, where they had a clear view of Mr. Trump.

Within minutes of Mr. Trump taking the stage, Mr. Steadman said, he noticed people pointing at the building adjacent to the tree they were sitting under. He went to take a closer look, then saw the shooter crawl across on the roof and pull out the black barrel of a gun. Mr. Steadman turned toward his daughter, who was about 30 feet away, and screamed, “He’s got a gun!”

Mr. Steadman said the shooter then rolled over onto his back, turned the opposite direction, and fired twice in the direction away from where Mr. Trump was speaking. Mr. Steadman said he could not see who the gunman was shooting at in that direction. After those first shots, Mr. Steadman started to run, and then heard another volley of gunshots.

He said he was stunned that he had been allowed to get so close. “We never should have been allowed to go where we were,” Mr. Steadman said. “Why that building was not secured, it makes no sense.”

It is still not clear when and how the gunman got on the roof of the warehouse.

Anthony Guglielmi, a Secret Service spokesman, said on Sunday that local law enforcement had been notified of a suspicious person by rally visitors before the event started.

In an interview on Monday, he said that local police officers were in contact through radio with the agency before the shooting, including about the concerned warnings from the passers-by.

“There were radio communications between the Secret Service and local authorities acknowledging that the local police were dealing with an incident, an issue of a suspicious person,” he said.

Mr. Guglielmi declined to provide more details, citing the pending investigations.

At 6:03 p.m. local time, Mr. Trump took the stage at the Butler Farm Show grounds, clapping and gesturing to the crowds as the song “God Bless the U.S.A.” played. By 6:09 p.m., videos analyzed by The New York Times show people in attendance were pointing at the roof of the warehouse. Two minutes later, the first shots rang out.

In aerial videos taken after the shooting, a ladder can be seen propped up against the building from which the shooter fired. An employee for AGR who works in the warehouse told The Times that employees there had never seen this ladder near the building before. It is unclear if this ladder was placed by the shooter, or law enforcement responders responding to the shooting.

Former Secret Service agents said that the agency begins planning for campaign events like this one days before, dispatching its advance team to survey the site and meet the local authorities.

In Butler, the Secret Service first reached out to the local authorities about the rally on July 5 — eight days beforehand — and held its first meeting with them on July 8, according to Steve Bicehouse, the county emergency services director.

Donald Mihalek, a former Secret Service agent who worked in the protective details for President George W. Bush and former President Barack Obama, said the service’s standard procedure is to finalize with the local authorities a security plan before the event, known as a “preliminary survey.”

The survey is then approved by a Secret Service supervisor. Leadership of other local law enforcement agencies also can weigh in, Mr. Mihalek said.

“Supervisors get briefed on the plan and check to see if there’s any gaps and try to fill those gaps. And once that’s done it’s game time,” Mr. Mihalek said.

Mr. Mihalek said the service tends to be stretched thin during campaigns, when they must protect both the sitting president and candidates simultaneously, he said.

For events like this one, the Secret Service divides the security zone into three sections: an inner perimeter staffed by the Secret Service, a middle perimeter that includes checkpoints staffed by both the Secret Service and local law enforcement and an outer perimeter typically policed by local law enforcement.

In towns that have previously hosted Trump rallies, police chiefs said that the Secret Service has the ultimate authority over where the security perimeters are set.

Michael Caron, chief of the Windham, N.H., police department, said his team had worked three presidential events, including a Trump rally in 2023 at the town’s high school. For that event, Mr. Caron said the Secret Services advance team contacted him about a week ahead to inform him of the location. During the team’s visit, the Secret Service discussed the site with local law enforcement officials, as well as contingency plans and what kind of assets and manpower the agency could provide.

“If we don’t have enough, we amend the plan and do what we can,” Mr. Caron said.

Vern Thomas, a captain at the Derry police department, worked with the Secret Service for a rally held in New Hampshire in 2023. Mr. Thomas said his department’s responsibility was not protection of the dignitary. Rather, officers focused their efforts on the surrounding areas, traffic and crowd control.

“We will walk through with the Secret Service, but it’s up to them to decide what is a threat and what’s not,” Mr. Thomas said.

Mr. Guglielmi said that in Butler, local law enforcement would have been responsible for sweeping the warehouse before the event started.

Former law enforcement officials with ties to the Secret Service have said the agency should have ensured that the building where the would-be assassin was positioned was covered by the federal agency’s security perimeter — in other words, by the protectors of the former president.

Jeffrey James, a 22-year veteran of the Secret Service, said that counter-sniper teams are trained to shoot from 1,000 yards away — and to scan areas out to that distance. He said close-up scans are typically done by those on foot.

The gunman in Butler County was only roughly 200 yards from Secret Service snipers. Mr. James said that might have been one reason that counter-sniper teams were slow to detect him.

“As a sniper, you’re not expecting anybody to be that close,” Mr. James said.

Hamed Aleaziz, Campbell Robertson, Aric Toler and Peter Baker contributed reporting. Kitty Bennett contributed research.

It was nighttime on Kooragang Island north of Sydney, Australia, when the high-pitched shrieking started.

John Gould, an ecologist at the University of Newcastle conducting postdoctoral research on the declining population of green and golden bell frogs, raced toward the chilling sounds. There, in a pond he had been surveying, he spotted a scene that might have fit in an amphibian reboot of a Hannibal Lecter movie: A large female frog was chomping down on the hind leg of a male while slowly pulling him into a hole.

“The male frog was trying really hard to prevent this from happening,” Dr. Gould said.

The act of apparent cannibalism was the first between adults recorded in this species, and it gave Dr. Gould an appetite to learn more about the topic. Ultimately, he believes that when a female green and golden bell frog isn’t pleased by the song of a male, she might opt to turn him into a meal.

The females “are almost the ultimate predators for males,” Dr. Gould said, because their ears are perfectly in tune to the calling of their would-be beaus.

Cannibalism is well known among amphibians. But usually it is the youngest frogs, toads or salamanders that end up as dinner. The tadpoles of various species eat smaller tadpoles, for example, to get ahead in life. In some cases, this happens regularly between siblings. In others, adults sometimes cannibalize eggs or larvae — researchers recently discovered that hellbender fathers may eat their young when faced with suboptimal water conditions.

But adult-on-adult cannibalism has seldom been witnessed. For a study published last month in the journal Ecology and Evolution, Dr. Gould scoured the literature and found only a couple of examples, many in the lab, of adult frogs’ cannibalizing other adults. Almost all of these occurred in cases where the females were bigger than the males. In green and golden bell frogs, for example, females can grow to about 2.75 inches in length while males usually max out at less than 2 inches.

Dr. Gould believes that a female may be able to tell whether a male is better for mating or eating based on the strength of his calls. This means males take a huge risk when trying to attract mates.

“You’ve really got to give props to the male frogs out there, that they are putting their lives on the line to reproduce,” Dr. Gould said. “Maybe there’s a reason why, males and females, you don’t often find them next to each other in ponds.”

David Pfennig, a biology professor at the University of North Carolina at Chapel Hill who was not involved in Dr. Gould’s research, called the study “a cool idea.” He has studied cannibalism among spadefoot toads — he has even seen tadpoles eat a toad that had already grown its legs.

But he would like to see more evidence of adult females cannibalizing males before agreeing that the phenomenon is more than occasional. While females may gain a clear benefit from cannibalizing males, there are also costs. Males might fight back, for example, or females could choke by biting off more than they could chew. Cannibalism can also spread disease in infected populations, Dr. Pfennig said.

Dr. Gould would also like to explore this idea more. And while tales of cannibalism don’t often have a happy ending, the male frog in Dr. Gould’s study lived to croak another day. After a struggle in which she pulled him deeper into the hole, he shrieked one more time and then managed to shake his leg free from the female’s mouth, hopping away to freedom.

An hour after the attempted assassination of former President Donald Trump, a false narrative that President Biden and his allies engineered the attack began to take hold.Credit…Doug Mills/The New York Times

Four minutes after the first report of a shooting at a rally for Donald J. Trump on Saturday, an anonymous account on X posted, “Joe Biden’s antifa shot President Trump.”

Within half an hour, another account on X with links to the QAnon conspiracy theory claimed without proof that the attack against Mr. Trump had most likely been ordered by the Central Intelligence Agency. Shortly after that, the far-right activist Laura Loomer posted on X about some recent remarks that President Biden made about Mr. Trump and then wrote, “They tried to kill Trump.” She did not provide evidence.

An hour later, with official details of the assassination attempt still scant, the narrative that President Biden and his allies had engineered the attack on Mr. Trump was being amplified by Republican lawmakers, Russian sympathizers and even a Brazilian political scion. By the time 24 hours had elapsed, posts about the unverified claim had been viewed and shared millions of times.

The idea that President Biden was behind the shooting of Mr. Trump was perhaps the most dominant conspiracy theory to emerge after the attack in Butler, Pa., on Saturday. The unproven conjecture surfaced almost instantly, hardened into a narrative and then catapulted between platforms large and small, even as information about the incident was limited. It was a striking example of the speed, scale and stickiness of rumors on social media, which often calcify into accepted truth far more efficiently than efforts to debunk or pleas for restraint.

That the subject this time was Mr. Trump, who frequently claims to be victimized by powerful forces while demonizing his enemies, only helped fuel the conspiracy theory. Its acceleration was also enabled by years of distrust stemming from tales of shadowy cabals of elites — which Mr. Trump has called “the deep state” — engaged in nefarious plots.

“The result was a perfect storm of righteous fury, blame-casting and conspiratorialism, at a moment when absolutely everyone was paying attention,” said Emerson Brooking, a resident senior fellow at the Atlantic Council’s Digital Forensic Research Lab, who studies online ecosystems.

Baseless claims of a left-sanctioned hit job on Mr. Trump were only part of “a massive online spread of false claims” about the shooting, according to the Institute for Strategic Dialogue, a nonprofit research group. References to false assassination narratives amassed more than 100 million views in 24 hours on X alone, the group said on Monday. That far exceeded the 35.1 million views for content related to false flag rumors and other conspiracy theories after a school shooting in Uvalde, Texas, in 2022.

Other unsubstantiated theories about the shooting were fueled in part by left-wing accounts, including that Mr. Trump had deliberately staged the shooting to improve his election chances, slashing his ear with a hidden razor, popping a concealed blood capsule or otherwise fabricating a fake gunshot wound. Fingers were also pointed at other imagined culprits, including the Israeli intelligence agency Mossad, Jews, trans people and Ukrainians.

But the unverified story line that President Biden and the Democrats were responsible stood out. According to the data firm PeakMetrics, the largest portion of discussion about the shooting on X and Telegram in the first seven hours — about 17 percent — involved expressions of solidarity and prayers for Mr. Trump. The next largest chunk, about 5 percent, accused Democrats of instigating the violence.

On July 12 and July 13 — the day of the shooting — there were 83,000 mentions on X of the phrase “inside job,” a 3,228 percent increase compared to the 48-hour period immediately prior, according to NewsGuard, which monitors online misinformation.

In a statement, a Biden campaign official said that after “this horrifying attack, anyone — especially elected officials with national platforms — politicizing this tragedy, spreading disinformation, and seeking to further divide Americans isn’t just unacceptable — it’s an abdication of leadership.”

The Trump campaign did not respond to a request for comment.

Adam Berinsky, a political science professor and misinformation expert at the Massachusetts Institute of Technology, said the rapid spread of conspiracy theories online reflected widespread political division.

“It says a lot about our current political moment that the politicization at the extremes is the natural default,” he said.

The timeline of the conspiracy theory focused on Mr. Biden and the Democrats’ culpability was documented by think tanks, private companies that monitor misinformation and research groups, including Advance Democracy, the Anti-Defamation League, the Atlantic Council and Cyabra.

The first signs of that unproven idea emerged minutes after gunshots sounded at Mr. Trump’s rally on Saturday.

Some of the conservative voices who lodged the accusations against the president and other Democrats have long histories of aggressive rhetoric themselves. Ms. Greene repeatedly called for executing Democrats before she was elected to Congress. Mr. Collins has endorsed violence toward immigrants. Several, including Ms. Greene and Mr. Vance, are scheduled to speak at the Republican National Convention in Milwaukee this week.

Outside the convention on Monday, Senator Steve Daines, a Republican of Montana, said the speculation online was “not helpful,” adding that “I see no evidence of” Mr. Biden or other Democrats inciting violence.

The conspiracy theories have since continued evolving.

One strain focused on accusations that Mr. Biden’s team had rejected earlier requests to bolster Mr. Trump’s protective detail, which have been denied by a Secret Service spokesman.

Video clips of Candace Owens, a conservative political commentator, declaring that the shooter “was allowed to scale that roof” have also drawn hundreds of thousands of likes on TikTok and Instagram. Similar claims surfaced on the video platform Rumble.

By Monday, some social media accounts were hawking merchandise promoting the conspiracy theories. T-shirts with images of a bloodied Mr. Trump raising his fist, with the words “Not Today Deep State,” were on sale on Truth Social. On TikTok, baseball caps with “STAGED,” using the same image of Mr. Trump, were also on offer for $25.

A version of this article appears in print on  , Section A, Page 1 of the New York edition with the headline: First, Gunshots. Moments Later, Disinformation.. Order Reprints | Today’s Paper | Subscribe