WASHINGTON — The U.S. Secret Service is investigating how a gunman armed with an AR-style rifle was able to get close enough to shoot and injure former President Donald Trump at a rally Saturday in Pennsylvania, in a devastating failure of one of the agency's core duties.

The FBI on Sunday identified the shooter as Thomas Matthew Crooks, 20, of Bethel Park, Allegheny County.

The gunman, who officials said was killed by Secret Service personnel, fired multiple shots at the stage from an “elevated position outside of the rally venue,” the agency said.

An Associated Press analysis of more than a dozen videos and photos taken at the Trump rally, as well as satellite imagery of the site, shows the shooter was able to get astonishingly close to the stage where the former president was speaking. A video posted to social media and geolocated by the AP shows Crooks' body lying motionless on the roof of a manufacturing plant just north of the Butler Farm Show grounds, where Trump's rally was held. A different image shows Crooks wearing a gray T-shirt with a black American flag on the right arm, and a bloody wound to his head.

The roof was fewer than 164 yards from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. For reference, 164 yards is a distance at which U.S. Army recruits must hit a human-sized silhouette to qualify with the M16 assault rifle in basic training. The AR-style rifle, like that of the gunman at the Trump rally, is the semiautomatic civilian version of the military M16.

President Joe Biden said Sunday he's directed an independent review of the security at the rally. Biden said he has also directed the U.S. Secret Service to review all security measures for the Republican National Convention, which begins Monday in Milwaukee.

Biden urged Americans not to make assumptions about the motive of the shooter. He said investigators are working swiftly to investigate the attack.

“Unity is the most elusive goal of all,” he said, but “nothing is more important than that right now.”

Calls for an investigation came from all sides.

Rep. Mark Green, a Tennessee Republican who chairs the House Committee on Homeland Security, sent a letter to Homeland Security Secretary Alejandro Mayorkas on Sunday raising questions about the shooting and demanding information about the former president's Secret Service protection.

“The seriousness of this security failure and chilling moment in our nation's history cannot be understated,” Green wrote.

The Secret Service did not have a speaker at a news conference Saturday night where FBI and Pennsylvania State Police officials briefed reporters on the shooting investigation. FBI Special Agent in Charge Kevin Rojek said it was “surprising” that the gunman was able to fire at the stage before he was killed.

Members of the Secret Service's counter-sniper team and counterassault team were at the rally, according to two law enforcement officials. The officials spoke on condition of anonymity because they weren't authorized to discuss details of the investigation.

The heavily armed counterassault team, whose Secret Service code name is “Hawkeye,” is responsible for eliminating threats so that other agents can shield and take away the person they are protecting. The counter-sniper team, known by the code name “Hercules,” uses long-range binoculars and is equipped with sniper rifles to deal with long-range threats.

Mayorkas said his department and the Secret Service are working with law enforcement to investigate the shooting. Maintaining the security of presidential candidates and their campaign events is one of the department's “most vital priorities,” he said.

“We condemn this violence in the strongest possible terms and commend the Secret Service for their swift action today,” Mayorkas said. “We are engaged with President Biden, former President Trump and their campaigns, and are taking every possible measure to ensure their safety and security.”

Green also noted reports that the Secret Service had rebuffed requests from the Trump campaign for additional security. A spokesman for the Secret Service, Anthony Guglielmi, said on X Sunday that those allegations were “absolutely false” and that they had added resources and technology as the campaign's travel increased.

Green said he would be talking with Secret Service Director Kimberly Cheatle on Sunday.

Former top Secret Service agents told The AP that Crooks should never have been allowed to gain access to the roof, and the agency will have to figure out how that happened. They said such a lapse could have been caused by officers neglecting their posts or a flaw in the event's security plan.

The agency is “going to have to go through the security plan and interview a number of people from the director on down” to figure out what went wrong, said Stephen Colo, who retired in 2003 as an assistant director after a 27-year career in the service.

Colo said presidential candidates and former presidents don't typically get the same level of protection as the sitting president. In fact, Colo said he was surprised that the agency had staffed the event with a counter-sniper team. Such a valuable resource — there are not many of those highly trained agents — is usually reserved for the president. Candidates don't usually get such teams.

Timothy McCarthy, a former agent who retired from the agency in 1994, said the Secret Service “better be doing a deep dive into what happened there and doing whatever it takes to figure it out” because the gunman should not have been able to occupy such a vantage point.

“How did that person get up on that building?” said McCarthy, 75, who in 1981 took a bullet when President Ronald Reagan was shot outside the Washington Hilton Hotel. “How did that happen? I mean, that's the key to the entire thing. And what measures were put in place to prevent it?”

James Comer, a Kentucky Republican who is the House Oversight Committee chairman, said he contacted the Secret Service for a briefing and called on Cheatle to appear for a hearing. Comer said his committee will send a formal invitation soon.

“Political violence in all forms is un-American and unacceptable. There are many questions and Americans demand answers,” Comer said in a statement.

U.S. Rep. Ritchie Torres, a New York Democrat, called for investigating “security failures” at the rally.

“The federal government must constantly learn from security failures in order to avoid repeating them, especially when those failures have implications for the nation,” Torres said.

Wisconsin Gov. Tony Evers, a Democrat, posted on X that he and his staff are in contact with security planning coordinators ahead of the Republican National Convention set to begin Monday in Milwaukee. “We cannot be a country that accepts political violence of any kind — that is not who we are as Americans,” Evers said.

The FBI said it will lead the investigation into the shooting, working with the Secret Service and local and state law enforcement.

Attorney General Merrick Garland said the Justice Department “will bring every available resource to bear to this investigation.”

“My heart is with the former President, those injured, and the family of the spectator killed in this horrific attack,” Garland said in a statement. “We will not tolerate violence of any kind, and violence like this is an attack on our democracy.”