Gunman Is Said to Have Acted Alone, but Motives Remain Unclear

The 20-year-old gunman who tried to assassinate former President Donald J. Trump at a rally in Pennsylvania appears to have acted alone, F.B.I. officials said on Sunday, but investigators remain unsure of his motives and political beliefs and have not yet been able to determine what evidence might be on his cellphone.

Agents found what officials described as a “rudimentary” explosive device in the gunman’s vehicle, and possible explosives were also found at his residence, according to a person with knowledge of the investigation.

F.B.I. officials confirmed that the gunman’s father had legally purchased the AR-15-type semiautomatic rifle used in the shooting. But they said it was not clear whether the father gave his son the weapon or whether he took it without permission.

Kevin Rojek,theF.B.I. special agent in charge in Pittsburgh, said the family was cooperating with the investigation.

Dozens of federal investigators scrambled to determine why the gunman, Thomas Matthew Crooks of Bethel Park, Pa., climbed atop a nearby building and squeezed off a volley of shots on Saturday evening that injured Mr. Trump, killed a man attending the rally with his family and left two other people at the site critically injured before he was killed by a Secret Service sniper.

F.B.I. officials said Mr. Crooks did not have a history of mental illness or criminal activity.

He does not appear to have left behind any written statement that could easily explain his motivations or provide clues to any external connections or influences, according to a senior law enforcement official.