Trump rally gunman purportedly left message on gaming platform before shooting: Sources

Investigators working to determine a motive behind the attempted assassination of former President Donald Trump 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 also found internet searches for both Trump and President Joe Biden on the phone belonging to Crooks, sources familiar with the investigation told ABC News. Crooks had searched for the dates of Trump's rally in Butler, Pennsylvania, and for those of the Democratic National Convention in Chicago, the sources said.

RELATED: Secret Service spotted shooter on roof 20 minutes before gunfire erupted at Trump rally

The suspect's phone was one of several devices that law enforcement investigators had collected as they worked to piece together a timeline of the assassination attempt against the former president during a Saturday rally.

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

Cohen, who specializes in the phenomenon of mass shooters, added that investigators may never determine a single or precise motive for the attack. Sources familiar with the investigation have told ABC News that a search of the suspect's phone history has revealed no indication of Crooks' political views or his motive for the shooting. Investigators have also found no digital footprint from the suspect suggesting any affiliations with international or domestic terrorist groups. They have also been unable to identify any ideological nexus to Trump and/or Biden, according to sources.

"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. As with those incidents, the warning signs were there, they were just not recognized," Cohen said. "The threat was real but people around him did not understand what they were witnessing or how it would play out last Saturday."

RELATED: What Thomas Matthew Crooks did in hours leading up to assassination attempt on Trump

The FBI early on Sunday had identified Crooks as the suspect. The U.S. Secret Service said snipers had killed Crooks at the scene. Firefighter Corey Comperatore, a dad who was attending the rally, was killed, and two other bystanders injured, officials said.

The searches compounded the questions that were swirling as investigators searched for a reason for the shooting, including a possible political motive. 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.

As investigators analyzed the suspect's phone, they looked at his search history, which included the queries for Trump and Biden. But that search history didn't immediately appear to reveal Crooks' political views, sources told ABC News.

Law enforcement officials investigating the assassination attempt told lawmakers on Wednesday that 20 minutes had passed between the time Secret Service snipers first spotted Crooks on a rooftop and the time the first shots were fired, according to several law enforcement officials and lawmakers briefed on the matter.

Editor's note: The story has been updated based on additional information from law enforcement.