A Proud Boys member who was among the first to cross the police line at the U.S. Capitol on Jan. 6 pleaded guilty on Wednesday and agreed to cooperate with the government — potentially against other members of the far-right extremist group, the authorities said…

The defendant, Matthew Greene, 34, of Syracuse, N.Y., was most likely the first member of the Proud Boys, a group that describes itself as “Western chauvinists,” to plead guilty to charges stemming from the riot, his lawyer, Michael Kasmarek, said on Wednesday…

Mr. Greene faces up to 25 years in prison on one count of conspiracy and one count of obstruction of an official proceeding, prosecutors said. But he could receive a sentence of 41 to 51 months because he agreed to cooperate with investigators, Mr. Kasmarek said…

After Mr. Greene returned home from the riot, he ordered more than 2,000 rounds of assault-rifle ammunition and told an “acquaintance” to be ready “to do uncomfortable things,” according to a motion that prosecutors filed in June. The F.B.I. seized his ammunition along with an AR-15 rifle and handguns while they were conducting a search at his home on Jan. 18.

Mr. Greene was arrested in April and agreed to share information with the government, Mr. Kasmarek said, adding that Mr. Greene “wants to take responsibility for what he did.”

Not any easy thing to do. He’s admitting to much more than crimes committed on the day. He will be expected to testify about the criminal conspiracy leading up to the insurrection attempt. As ex-military, his skill set could offer him an important place in this plot.