Gunman opened fire at Trump rally as witnesses say they tried to alert police

As federal investigators continue to probe how a gunman managed to climb atop a roof and fire a barrage of shots at former President Donald Trump in an apparent assassination attempt on Saturday, multiple witnesses said they tried to point out the suspect to police and Secret Service agents before gunfire erupted.

Roughly eight minutes after Trump took the stage at a campaign rally in Butler Township, Pennsylvania, witnesses said they saw the alleged gunman shimmying up the sloping roof of the American Glass Research building outside the security perimeter of the rally.

The building is within 400 feet of the rally podium where Trump was speaking and was being used by local police as a staging area when the gunman got on the roof, U.S. Secret Service Director Kimberly Cheatle said in an exclusive interview with ABC News.

Witness Ryan Knight, who was working at the rally selling pizza slices and bottled water with his boss, said he was standing outside the security perimeter at a fence line when someone standing next to him said "This guy's got a gun" and pointed.

"I look over at the AGR building and there was a guy prone, laying down on the AGR roof," Knight told ABC News. "At that point, I look over and the guy starts taking shots at the president. I am losing my mind. I'm thinking, 'Get down on the ground.'"

According to a timeline of events pieced together by the ABC News video verification team, Trump took the stage at 6:03 p.m. local time to loud applause and at 6:05 p.m. began speaking: "This is a big crowd. This is a big, big beautiful crowd."

Knight, who says he is a registered Republican and a supporter of the former president, explained that once Trump took the stage, he and his boss walked over to the fence line about 50 feet from the AGR building to listen to the speech.

Knight said the rally was initially joyful, with attendees yelling their support for Trump.

"We were just having a great time. I mean, everybody was in happy spirits," Knight said.

But at 6:09 p.m., bystander video taken outside the security perimeter near the fence line and reviewed by ABC News picked up someone yelling, "He's got a gun!"

At 6:10 p.m., another bystander video reviewed by ABC News showed a police officer walking around the AGR building investigating reports of a suspicious person reported to be in the area.

Around that same time, Trump turned his head to the right to look at a Jumbotron flashing immigration statistics, according to video footage. About a minute later, the staccato of gunshots rang out and the GOP presidential nominee suddenly clutched his right ear and went to the ground after apparently noticing blood on his hand, according to the video.

A New York Times photographer took a photo that appeared to capture the path of a bullet whizzing by Trump's head. Three rallygoers were shot in the incident, including 50-year-old volunteer firefighter Corey Comperatore who was killed shielding his family from the gunshots, officials said. The two other rallygoers were listed in stable condition.

Secret Service agents rushed to Trump and covered him with their bodies as chaos erupted.

Mike and Amber DiFrischia of Adamsville, Pennsylvania, told ABC News that they arrived at the rally around 5:53 p.m. and decided to sit in the shade outside the security perimeter until Trump began to speak. The couple said they moved up to the fence line to listen to Trump. Amber DiFrischia said it was the first time she had attended a Trump rally.

"We were only at the fence for maybe five minutes, Trump came on, and then shortly after he started speaking, my wife said, 'Michael what is going on behind us?'" Mike DiFrischia said. "We turned around and we saw people pointing and running and that's when we walked away from the fence and noticed that he [the gunman] was shimmying up the roof."

At first, he said thought it was someone just trying to get a better view of Trump.

"Then the guy behind me said, 'The guy's got a gun.' And as soon as he said 'gun,' everybody started running and screaming," Mike DiFrischia said.

He said he and others could see the gunman "perfectly" on the roof because they were a short distance away, but police couldn't see him immediately due to being at the base of the building.

"The law enforcement was running around. It was pretty hectic. They could not see him, the police officers. They were too close to the building," Mike DiFrischia said.

He said police officers were yelling, "Where is he at?"

"And we were trying to explain to them, he's right there," Mike DiFrischia said.

Amber DiFrischia added, "How could this happen so quickly?"

Butler County Sheriff Michael T. Slupe said law enforcement officials told him that a Butler Township police officer was vaulted onto the roof and confronted by the gunman who pointed his weapon at the officer.

"He dropped down because the gentleman was turning with the firearm," Slupe told ABC News, adding that the officer immediately radioed in about the gunman.

According to the ABC News video verification team's timeline, gunfire erupted around 6:11 p.m. Investigators believe the gunman immediately opened fire after being interrupted by the Butler Township police officer.

"I got on the ground and I'm watching him shoot," Knight told ABC News of the gunman. "I could see the smoke coming out of the barrel of his gun."

Video of the mayhem showed a Secret Service counter-sniper firing at the gunman from a rooftop behind the stage where Trump was giving his speech.

"I was scared for my life," said Knight, explaining that he and other spectators were standing in what was an open field with nowhere to take cover.

He said he heard a shot that sounded louder than the initial gunfire and saw the gunmen's head kick back as he was shot.

"My main thought is why the Secret Service was not on top of that building," Knight said. "There should have been people on top of it. I don't know how or why that got missed."

Cheatle, the Secret Service director, told ABC News chief justice correspondent Pierre Thomas on Monday that local authorities were tasked with securing the building where the alleged shooter fired the shots and confirmed that local police were present inside the building while the shooter was on the roof.

"In this particular instance, we did share support for that particular site and that the Secret Service was responsible for the inner perimeter," Cheatle said. "And then we sought assistance from our local counterparts for the outer perimeter. There was local police in that building -- there was local police in the area that were responsible for the outer perimeter of the building."

President Joe Biden said Sunday that he has ordered an independent investigation of the security provided at Saturday's rally.

U.S. Secretary of Homeland Security Alejandro Mayorkas, who oversees the Secret Service, told George Stephanopoulos on "Good Morning America" on Monday that the 20-year-old gunman, identified as Thomas Matthew Crooks, should never have been able to get a clear line of sight on the former president.

"George, a direct line of sight like that to the former president of the United States should not occur," Mayorkas said.

Cheatle told ABC News in the interview on Monday that the Pennsylvania rally shooting was "unacceptable."

"I don't have all the details yet, but it was a very short period of time," she said about the shooter being seen by witnesses before he opened fire. "Seeking that person out, finding them, identifying them, and eventually neutralizing them took place in a very short period of time, and it makes it very difficult."