Jeffrey Finley photo from Justice Department.
CNN  — 

The leader of the West Virginia chapter of the Proud Boys pleaded guilty on Wednesday to illegally entering the US Capitol grounds on January 6, 2021.

Jeffrey Finley, 29, faces up to one year in prison.

The plea could be a significant step in one of the Justice Department’s most crucial investigations related to the attack, as prosecutors home in on what Proud Boys leaders had planned as they descended on the Capitol. Finley has corroborated evidence prosecutors may use in other cases.

According to the plea agreement read in court, Finley was among the first group to breach the Capitol grounds. Though Finley was not charged with other members of the Proud Boys, prosecutors say he was photographed with some of the group’s leaders marching on the grounds.

Finley also wore an earpiece during the riot. Prosecutors have alleged that Proud Boys members used radios – and some wore earpieces – to communicate on January 6. And Finley took part in Telegram message channels that prosecutors have relied upon in making their cases.

Just after 2 p.m. that day, Finley moved with the mob to the Upper West Terrace and messaged a Telegram channel titled “Boots on the Ground.” Prosecutors have alleged that channel was used by Proud Boys leaders to communicate, including reposting instructions from leader Enrique Tarrio.

Almost two hours later, Finley sent another message to the “Boots on the Ground” channel saying, “I just got out myself” and “we literally can’t get back in… if you guys come out you aren’t getting back in.”

In the days after the riot, according to the plea agreement, Finley deleted photographs, videos and messages with other Proud Boys at the Capitol. Finley instructed other members to do the same.

“No talks about DC,” Finley said in one message.

Last month, the Justice Department charged Tarrio with conspiracy. He pleaded not guilty on Tuesday.

Prosecutors have indicated in court that there is new evidence that could lead to additional charges or arrests in relation to that case.