There Is No Immediate Evidence That the Gunman Had Help, Investigators Say

Investigators are scrambling to determine if the man who attempted to assassinate former President Donald J. Trump acted alone, what motivated him to undertake political violence and how he came to possess a semiautomatic rifle and materials for several explosive devices.

Dozens of agents with the Federal Bureau of Investigation and 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.

While the effort was proceeding at a breakneck pace, one senior U.S. law enforcement official cautioned that investigators were in “uncharted territory” and would not rule out any potential scenario until all leads had been exhausted.

As of Sunday morning, they had found no evidence so far indicating that the gunman, Thomas Matthew Crooks, a 20-year-old from Bethel Park, Pa., was part of a larger plot.

But law enforcement officials cautioned that the investigation was extremely fluid, and they had yet to extract and analyze information from Mr. Crooks’s electronic devices for potential connections to other people.

The F.B.I. has identified Mr. Crooks, a slender young man with long hair and glasses, as the person responsible for firing an AR-15-type rifle at the president as he spoke to supporters at a rally near Pittsburgh, grazing Mr. Trump on the ear, killing a bystander and injuring several others at the rally in Butler, Pa.

It could have been much worse, despite the loss of life and the convulsive shock through the country’s political system. Law enforcement officials found materials that could be used to make two explosive