A lawmaker from West Virginia and a man who broke into Speaker Nancy Pelosi’s office and posed at her desk were among those arrested on charges related to the siege at the Capitol, federal law enforcement officials announced on Friday as they promised an exhaustive investigation into the violence. The authorities also found 11 Molotov cocktails and a semiautomatic rifle in the truck of a 70-year-old man from Alabama who was also arrested, according to prosecutors. He also had two handguns. Hundreds of prosecutors and F.B.I. agents have been assigned to work the investigation and were pursuing dozens of cases, Ken Kohl, a prosecutor in the U.S. attorney’s office in Washington, said in a briefing with reporters. “We are far from done,” added Steven M. D’Antuono, who runs the F.B.I.’s Washington field office.