WASHINGTON (AP) — Authorities want to know how a shooter was able to get on top of a roof so close to where former President Donald Trump was speaking and open fire.

That is one of the focuses of the investigation into Saturday’s assassination attempt on Trump as Republicans gather in Milwaukee for their national convention and President Joe Biden implored Americans to reject political violence.

Here’s a look at what we know so far:

How did a gunman get so close to Trump?

The U.S. Secret Service is investigating how the gunman armed with an AR-style rifle was able to get on a nearby roof and shoot and injure the former president at a rally Saturday in Pennsylvania.

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. Trump was bloodied and says he was “shot with a bullet that pierced the upper part of my right ear.” A spectator was killed.

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.

The roof was fewer than 150 meters (164 yards) from where Trump was speaking. Biden has directed an independent review of the security at the rally.

What is known about the Trump rally shooter?

Thomas Matthew Crooks, the 20-year-old nursing-home employee from suburban Pittsburgh who tried to shoot Trump was, a registered Republican who packed explosives in the vehicle he drove to the campaign rally an hour from his home.

Authorities said it was attempted assassination but haven’t determined what motivated Crooks to try to kill Trump.

Law enforcement officials were working Sunday to learn more about the young man from Bethel Park, Pennsylvania, to determine what motivated him.

The FBI has not yet identified any underlying ideology or threatening writing or social media posts from Crooks, who had no past criminal cases against him, according to public court records. The FBI believes Crooks acted alone and the shooting was being investigated as a potential act of domestic terrorism.

What do we know about the other victims?

Gov. Josh Shapiro on Sunday said the man killed at the Trump rally, Corey Comperatore, “dove on his family to protect them.”

“Corey died a hero,” the governor said. Comperatore, 50, was a former fire chief.

Pennsylvania State Police identified two other men who were shot as David Dutch, 57, of New Kensington, and James Copenhaver, 74, of Moon Township. Both men remained hospitalized and were listed in stable condition, state police said.

Biden decries violence in national address

In a prime-time national address, the president said Sunday that political passions can run high but “we must never descend into violence.”

“We can do this,” Biden implored, saying the nation was founded on a democracy that gave reason and balance a chance to prevail over brute force.

“American democracy — where arguments are made in good faith. American democracy where the rule of law is respected. Where decency, dignity, fair play aren’t just quaint notions, they’re living, breathing realities.”

Trump arrives in Milwaukee as RNC goes on

The former president is Milwaukee for the Republican National Convention.

Trump’s airplane touched down Sunday. The four-day event kicks off Monday with thousands of Republicans coming together to formally choose him as their 2024 presidential nominee.

His aides said he was in “great spirits” and doing well.

Saturday’s attack on Trump has put a heightened focus on safety and security of the event.

Trump said he was going to delay his trip by two days because of the attempted assassination “but have just decided that I cannot allow a ‘shooter,’ or potential assassin, to force change to scheduling, or anything else.”

Have there been other presidential assassination attempts?

Saturday’s attack was the most serious attempt to assassinate a president or presidential candidate since Ronald Reagan was shot in 1981, but there have been multiple instances of political violence targeting U.S. presidents, former presidents and major party presidential candidates. Four U.S. presidents have been assassinated – Abraham Lincoln, James Garfield, William McKinley and John Kennedy.