(NewsNation) — The FBI told lawmakers that the agency is still working to determine what motive Thomas Matthew Crooks had in attempting to shoot and kill former President Donald Trump.

Former Secret Service agent Paul Eckloff believes the best way to understand Crooks’ motive is to look at the “history of shooters in America.”

“If you look back at a history of mass shooters in public spaces, 25% had a diagnosed mental illness,” Eckloff said Thursday, during NewsNation’s special report on the assassination attempt. “That’s interesting because that’s a very low number. In this case, it appears he (Crooks) was diagnosed with a major depressive disorder, but you tend to look at changes in behavior.” NewsNation independently confirmed Crooks’ diagnosis.

Eckloff said that in the same way that the FBI can “forensically look at the site and find where the errors were” … “there are opportunities to intervene” in cases involving mass shootings.

“We’ll need to go back and draw a timeline of his behaviors, and certainly, his internet searches are part of that. … Him researching assassins is definitely a sign. But as we know, America’s fascinated with assassins,” Eckloff added.

According to the FBI, on July 6, the day Crooks registered for the Butler rally, he also ran a Google search on his computer asking how far Lee Harvey Oswald was from President John F. Kennedy when Kennedy was assassinated in Dallas in 1963.

That online search, FBI Director Christopher Wray said, provides insight into Crooks’ mindset in the days leading up to the assassination attempt. Wray also testified the online searches began to focus more on Trump and the Butler rally.

Wray testified Crooks flew a drone about 200 yards from the stage where Trump was set to speak more than two hours before the shooting took place.

Crooks, a 20-year-old nursing home employee who graduated from community college last spring, had searched online for information about Trump and Biden and had photos of both men on his phone, lawmakers told the Washington Post.

The newspaper also reported that photos of U.S. Attorney General Merrick Garland and a member of the British royal family were also on Crooks’ phone. Wray testified that Crooks was using an encrypted messaging app on his phone.

A search of Crooks’ phone by the FBI also determined that he had conducted online searches about a shooting in which a teenage gunman, Ethan Crumbley, killed four high school students in Michigan as well as information about major depressive disorder, the rally in Butler, and the Democratic National Convention.

NBC News reported other online searches run by Crooks in the months leading up to the rally included one on explosive materials and chemical compounds as well as improvised explosive devices and the Department of Homeland Security.

The report indicated there were more than 14,000 links and photos found on Crooks’ phone, including one photo of the rally that was taken before Trump took the stage.

Click here to watch the full NewsNation Special Report: The Plot to Kill Trump.

NewsNation’s Jeff Arnold contributed to this report.