But an initial anatomy of the siege by The New York Times revealed numerous failures. The chaos showed that government agencies have no coordinated plan to defend against an attack on the Capitol — especially one specifically aimed at powerful elected officials — though law enforcement agencies have for years raised alarms about the growing threat of domestic terrorism. QAnon, an online conspiracy group that was well represented among the crowd, has been labeled a domestic terrorist threat by the F.B.I.

Federal agencies and the Capitol Police appeared to issue no serious warnings in the days leading up to the riots that the gathering could turn violent, despite countless posts on right-wing social media sites pledging confrontation and even bloodshed.

The Department of Homeland Security invited local law enforcement agencies to its situation room — held online during the pandemic — only the day before the riots, which some security experts said was far too late.

Poor planning and communication among a constellation of federal, state and local law enforcement agencies hamstrung the response to the rioting. Once the Capitol building was breached, a patchwork group of reinforcements was forced to try to navigate a labyrinthine complex of unfamiliar passages and byways that would prove dangerous.