Mr. Trump’s Twitter post in the early hours of Dec. 19, 2020, was the first time he publicly urged supporters to come to Washington on the day Congress was scheduled to certify the Electoral College results showing Joseph R. Biden Jr. as the winner of the presidential vote. His message — which concluded with, “Be there, will be wild!” — has long been seen as instrumental in drawing the crowds that attended a pro-Trump rally on the Ellipse on Jan. 6 and then marched to the Capitol.

But the Justice Department’s criminal investigation of the riot and the parallel inquiry by the House select committee have increasingly shown how Mr. Trump’s post was a powerful catalyst, particularly for far-right militants who believed he was facing his final chance to reverse defeat and whose role in fomenting the violence has come under intense scrutiny.

Extremist groups almost immediately celebrated Mr. Trump’s Twitter message, which they widely interpreted as an invitation to descend on the city in force. Responding to the president’s words, the groups sprang into action, court filings and interviews by the House committee show: Extremists began to set up encrypted communications channels, acquire protective gear and, in one case, prepare heavily armed “quick reaction forces” to be staged outside Washington.