Overwhelmed police officers met and battled a frenzied sea of rioters during the turmoil at the US Capitol that turned the seat of democracy into a battlefield.

Police chiefs across the nation are now facing the inconvenient truth that members of their own ranks were among the crowd facing other law enforcement officers.

There are at least 13 off-duty law enforcement officers accused of engaging in the riot, a tally that may expand as authorities continue to pore over videos and documents to locate participants.

Police officials turn to the FBI on their own and take the striking step of reminding officers in their units that they could be forced away by criminal wrongdoing from the force and behind bars.

The reckoning inside police forces comes as preparations for fresh protests this weekend and are solidifying on inauguration day, with US authorities warning of the potential for unrest in state capitals. Participants are expected to protest the outcome of the elections that made Joe Biden elected president.

The announcement that officers took part in the chaos was the latest blow for the image of law enforcement, coming on the heels of a year in which national demonstrations were spurred by police brutality and activists called for slashing police funding.