The man who attempted to assassinate former President Donald Trump at a campaign rally on Saturday called the date of the attack his "premiere" in an online post, a law enforcement official and an additional source familiar with a briefing given to U.S. lawmakers on Wednesday told CBS News.

The shooter had an account on an online gaming platform, on which he posted: "July 13 will be my premiere." 

The message was found as investigators seek to uncover a motive for the shooting. 

The gunman, identified as 20-year-old Thomas Matthew Crooks of Bethel Park, Pennsylvania, opened fire from the roof of a building near the rally, hitting Trump in the right ear. One attendee, identified as Corey Comperatore, 50, was fatally struck. Two other attendees, James Copenhaver, 74, and David Dutch, 57, were injured. 

The gunman searched for images of Trump and President Biden ahead of the shooting, according to law enforcement sources. The sources also told CBS News the gunman did searches about Trump's public appearances and dates, and about the Democratic National Convention. 

The gunman visited the site of Trump's rally at least one time before the shooting, sources said.

The shooter also did searches for FBI Director Christopher Wray, Attorney General Merrick Garland and members of the British royal family. He also searched for information about major depression disorders, the law enforcement sources said. The shooter had two cell phones from which he conducted the searches. 

Investigators have still not determined a motive, ideology or set of political views that may have led to the shooting, the sources said. Wray told Congress on Wednesday that more than 200 interviews have been conducted during the investigation and more than 14,000 images have been reviewed. 

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• Trump Rally
• Donald Trump