“We’ve already arrested close to 500, and we have hundreds of investigations that are still ongoing beyond those 500,” Christopher A. Wray, the F.B.I. director, told the House Oversight Committee. His assurances of how seriously the agency was taking the attack by a pro-Trump mob came as lawmakers pressed him and military commanders on why they did not do more to prevent the siege despite threats from extremists to commit violence. “The threats, I would say, were everywhere,” said Representative Carolyn B. Maloney, a New York Democrat who is the chairwoman of the Oversight Committee. “The system was blinking red.”