The gunman in the assassination attempt on former U.S. President Donald Trump had searched online for events of both Trump and President Joe Biden and seen the Pennsylvania campaign rally where he opened fire last month as a "target of opportunity," a senior official of the Federal Bureau of Investigation (FBI) said Wednesday.

Thomas Matthew Crooks, who shot at Trump from a nearby roof before being killed by a Secret Service counter-sniper, did extensive research for an attack before the shooting and had looked at any number of events or targets, including the current and former presidents, said Kevin Rojek, the special agent in charge of the FBI's Pittsburgh field office.