When shots rang out at a campaign event for former President Trump, one attendee stepped up to help the wounded, including a volunteer firefighter who was killed by the gunman. 

Retired emergency physician Dr. James Sweetland recounted the horrific events on "Fox News @ Night," saying the shots sounded like firecrackers initially. 

"There were three pops. There was maybe a three or four-second pause, and then it was followed by four more pops," he said. 

"I, like everybody in the bleachers, turned to look to our right, over the field, and a line of trees to see where it was coming from. I then turned back to see Secret Service piling on President Trump. It quickly became a human pyramid of about six Secret Service agents." 

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The doctor then heard a woman scream for help nearby in the bleachers. 

"I heard a voice ring out, a woman from the stands, just 30 feet away down to my right and down on the benches, saying he's been shot. Help! He's been shot. He's down. Help him!"

Dr. Sweetland then rushed over to help 50-year-old Corey Comperatore.

"When I got to him, and I stumbled over people that were hunkered down, there was a large pool of blood. Unfortunately, there was a brain matter there. I did a brief examination of him. He had a bullet wound just above his right ear. And it looked to be fatal," he said.

After moving Comperatore onto the bleacher bench, Dr. Sweetland performed CPR but said the man had no pulse and was not breathing.

"I was performing CPR at the feet of his wife and daughter. I'll never forget the look in their face when I looked up at them. It was…of grief, being appalled and still kind of a look of hope. And I remember just blurting out to them, he's being taken where he can get help."

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Comperatore died shielding his wife and family from the gunfire after a 20-year-old man snuck onto a roof and started shooting at Trump during a rally in Butler, Pennsylvania.

The shooting also left two rally-goers injured while Trump narrowly survived with only a hit to the ear.

Law enforcement has identified the two other victims as James Copenhaver, 74, of Moon Township, Pa., and David Dutch, 57, of New Kensington, Pa. 

When the shots first rang out, Trump went down, surrounded by Secret Service agents. It was unclear at first whether Trump had been hit. 

"I thought the worst," Dr. Sweetland said. "I heard a roar from the crowd behind me. I didn't turn around, but I assumed that there was something going on behind me."

"I wasn't aware till later that those cheers were when he gave the fist bump up in the air, and that was what the crowd wanted to know that he was OK."

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The former president continued with plans to attend the Republican National Convention in Milwaukee despite the tragedy. Trump has reflected on the horrific close-call, telling ABC News he has rewritten what was supposed to be a "humdinger" of a speech after a shooter attempted to kill him on Saturday, and will now present one aimed at "unity."

The FBI is leading the ongoing investigation into the assassination attempt and the shooter, Thomas Matthew Crooks, 20.

Fox News' Brooke Singman, Greg Norman, Michael Ruiz, Lucas Tomlinson, Sarah Rumpf-Whitten, Anders Hagstrom, Gabriele Regalbuto, Michael Lee, Greg Wehner, Bradford Betz, Stepheny Price, Louis Casiano, Landon Mion and Elizabeth Pritchett contributed to this report.