As investigators continue to work to decipher a motive in the assassination attempt against former President Donald Trump, one expert said the 20-year-old gunman, Thomas Matthew Crooks, was likely motivated by a "combination of factors."

"What is beginning to emerge is a portrait of a troubled young person who turned to violence," said ABC News contributor John Cohen, the former head of intelligence at the Department of Homeland Security who specialized in the radicalization and the phenomenon of mass shooters.

"Investigators may never determine a single or precise motive for the attack. Likely, it was a combination of mental health issues, ideological beliefs and a sense of personal grievance -- the same combination of factors present in almost every school shooting and mass casualty attack over the past several years," Cohen said. "As with those incidents, the warning signs were there, they were just not recognized. The threat was real, but people around him did not understand what they were witnessing or how it would play out."

Sen. Marsha Blackburn, R-Tenn., and other Republican senators chased down Secret Service Director Kim Cheatle at the Republican National Convention Wednesday.

Multiple sources told ABC News that Cheatle was on the suite level of the convention hall, and when she was spotted, multiple senators chased her.

"This was an assassination attempt -- you owe the people answers, you owe President Trump answers," Blackburn said in reference to the shooting at Trump's rally on Saturday, according to a video.

Sen. John Barrasso, R-Wyoming, is heard accusing Cheatle of "stonewalling" them as she refused to respond to their repeated jeers, based on videos posted to the senators' social media.

Blackburn said Sen. James Lankford, R-Okla, and Sen. Kevin Cramer, R-N.D., also joined them in confronting the director.

"She can run, but she cannot hide," Blackburn said in a separate video. "Because the American people want to know how an assassination attempt was carried out on former President Donald Trump."

-ABC News’ Katherine Faulders and Allie Pecorin

PBS News has obtained video of former President Donald Trump’s Wednesday appearance during the movie screening of "Trump’s Rescue Mission: Saving America."

Trump told supporters of the shooting, “I got lucky, God was with me.”

"That was an amazing, horrible thing. Amazing thing, and in many ways it changes your attitude, your viewpoint towards life. And honestly, I think you appreciate God even more," he said.

"It wasn’t like it was a complete miss. It was pretty terrible that that can happen," Trump added.

-ABC News’ Lalee Ibssa, Kelsey Walsh and Soorin Kim

Investigators are examining a message left on a gaming platform site purported to have been authored by the suspect, according to multiple law enforcement sources briefed on the probe.

Investigators, according to multiple law enforcement sources, found a post on the gaming platform Steam that is credited to the suspected gunman, 20-year-old Thomas Matthew Crooks. The post reads, "July 13 will be my premiere, watch as it unfolds." Investigators are working to determine whether the post is legitimate.

Investigators are still working to determine a motive behind the attempted assassination of former President Donald Trump.

Crooks had been registered as a Republican voter, according to state records. But a $15 donation to a progressive group was also recorded under "Thomas Crooks" in January 2021, according to FEC records.

This post has been updated based on additional information from the FBI.

Authorities "have not identified an ideology associated" with the suspected shooter, Thomas Crooks, FBI Pittsburgh Special Agent in Charge Kevin Rojek told reporters on a call on Sunday.

"We're looking into his background, his day-to-day activities, any writings and social media posts that might help us identify what led to this shooting. And we have not seen anything threatening," Rojek said.

Crooks had no known mental illness history, he said.

Crooks’ family is cooperating with federal authorities, officials said.

Officials told reporters they don’t have any insights on what the suspect's actions were prior to opening fire at the shooting site, but said it's something they are investigating.

Robert Wells, the FBI's assistant director of the Counterterrorism Division, told reporters, "We are investigating this as an assassination attempt, but also looking at as a potential domestic terrorism act."

The FBI has received more than 2,000 tips, according to Rojek.

Crooks’ gun and phone will be transported to the FBI lab in Quantico, Virginia, officials said.

Rojek also confirmed there was a suspicious device that was found in the suspect’s car and said the FBI is still analyzing what it is. Another device was found in the suspect’s home, according to the FBI. Rojek described both the devices as rudimentary.

The FBI is seeing an increase of violent threats online in the aftermath of the assassination attempt, FBI Deputy Director Paul Abbate said.

"We've seen individuals go online and attempt to mimic or pose as the shooter who obviously is now deceased," Abbate said. "We're also focused on the continuing efforts, which were already substantial, given that there are national special security events on the upcoming conventions in Chicago, in Milwaukee, to work with Secret Service in the lead to play our part in protecting the people and the facilities and events there, as well."

FBI Director Christopher Wray called the assassination attempt an "attack on our democracy."

"An attempt to assassinate a presidential candidate can only be described as absolutely despicable and will not be tolerated in this country," Wray told reporters.

-ABC News’ Pierre Thomas, Aaron Katersky, Jack Date and Luke Barr