As scores of Proud Boys made their way, chanting and shouting, toward the Capitol on Jan. 6, one member of the far-right group was busy texting a real-time account of the march.

The recipient was his F.B.I. handler.

In the middle of an unfolding melee that shook a pillar of American democracy — the peaceful transfer of power — the bureau had an informant in the crowd, providing an inside glimpse of the action, according to confidential records obtained by The New York Times. In the informant’s version of events, the Proud Boys, famous for their street fights, were largely following a pro-Trump mob consumed by a herd mentality rather than carrying out any type of preplanned attack.

After meeting his fellow Proud Boys at the Washington Monument that morning, the informant described his path to the Capitol grounds where he saw barriers knocked down and Trump supporters streaming into the building, the records show. At one point, his handler appeared not to grasp that the building had been breached, the records show, and asked the informant to keep him in the loop — especially if there was any violence.