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, the F.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.
Records show that Mr. Crooks registered to vote as a Republican but that he also donated $15 to a group allied with Democrats. But little else is known about Mr. Crooks, a nursing home employee with glasses and long hair who has been described by former classmates as intelligent and unassuming.
He had an engineering degree from a nearby community college. He liked to play chess and video games and was learning how to code. Unlike most other people of his generation, he seemed to have a minimal presence online, leaving no telltale trail of public comments or social media posts, according to a review of his online activities by officials.
The shooting is being investigated as both an attempted assassination and an episode of domestic terrorism, officials said.
Attorney General Merrick B. Garland and Christopher A. Wray, the F.B.I. director, said on Sunday that they planned to be transparent and timely in providing updated information about the investigation because of its political significance and to counter misinformation already circulating on social media, including fake accounts purporting to come from Mr. Crooks.
“I want to reiterate that the violence that we saw yesterday is an attack on our democracy itself,” Mr. Garland said, echoing language he has used to describe the Jan. 6, 2021, attack on the Capitol by a pro-Trump mob.
“As Americans we must have no tolerance for it,” he added. “This must stop.”
But they are leading the investigation after years in which Mr. Trump and his allies have accused the Justice Department and the bureau of being deeply biased against him — and at a time when federal prosecutors are pursuing two criminal cases against the former president.
While the bureau and investigators with the Justice Department’s national security division have found no evidence that the shooting was part of a larger plot or connected to any foreign forces, a senior U.S. law enforcement official cautioned that investigators were in “uncharted territory” and said they would not rule out any possibilities until they had scrubbed Mr. Crooks’s devices and interviewed witnesses.
“At this time, the information that we have indicates that the shooter acted alone and that there are currently no public safety concerns at present,” Mr. Rojek said in a conference call with reporters on Sunday.
“We have not identified an ideology associated with the subject, but I want to remind everyone that we’re still very early in this investigation,” he added.
Mr. Wray said his investigators would “leave no stone unturned.”
Investigators were hoping that Mr. Crooks’s cellphone could provide information about his motives, beliefs and connections. F.B.I. officials in Pennsylvania were not able to break into the device and have shipped it to the bureau’s lab in Quantico, Va., where they hope to quickly overcome its password protection, Mr. Rojek said.
They have had limited access to some of his text messages, he said, but they have provided little insight into his motivations or beliefs. The explosive materials and the rifle were also sent to Quantico for study.
Dozens of federal investigators, working at a breakneck pace, are trying to develop a clearer picture of a young man who not only had little social media presence but also carried no identification on him at the time of the shooting and placed explosive devices in his car, impeding investigators, according to the F.B.I.
The F.B.I., which has received 2,000 tips in the case, is focused on tracking Mr. Crooks’s movements in the hours, days and weeks before the shooting. They have deployed a team on the ground to process physical evidence, and technical specialists to analyze fingerprints, DNA evidence, ammunition casings and digital devices.
Dozens of agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives, along with prosecutors from three U.S. attorneys’ offices in Pennsylvania, executed warrants and interviewed witnesses in an all-hands dragnet that included several other federal and local law enforcement agencies.
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