Graydon Young, 54, of Florida, pleaded guilty on Wednesday to conspiracy charges brought against him and 16 other members of the Oath Keepers, who prosecutors claim plotted to carry out violence on 6 January.
Mr Young pleaded guilty to two counts against him, conspiracy and obstruction of an official proceeding, both of which are felonies.
The former charge carries a maximum sentence of five years in prison, with the latter carrying a maximum of 20 years.
He also promised to cooperate with investigators as part of his plea deal. That cooperation will include his testimony before a grand jury and at trial. He will also have to give interviews to law enforcement personnel.
The presiding judge told Mr Young his estimated sentencing guideline was 63 to 78 months in prison. The judge said those estimates were influenced by the “obstructive nature” of his crimes, but that he was also granted credit for accepting responsibility by pleading guilty.
The judge also told Mr Young he could further reduce his sentence if the government finds his cooperation helpful.
As part of his plea agreement, Mr Young also agreed to pay $2,000 in restitution. The judge noted that the Capitol riot caused $1.5m in damages.
Mr Young is the first – and so far the only – individual targeted by a major conspiracy charge to plead guilty to his charges.
Mr Young's co-defendants in the conspiracy case are contesting the charges being brought against them.
Prosecutors in the case allege that Mr Young was part of “the stack”, which was a group of militants who formed a line to cut through the crowds in order to enter the Capitol quickly.
The man joined the paramilitary gang sometime before 6 January and was soliciting recruits on Facebook in December.
Mr Young was arrested in Tampa, Florida in February and was then turned over to US Marshals. He has been charged with six crimes, which include conspiracy and aiding and abetting the obstruction of the congressional proceeding.
Prior to the attack on the Capitol, Mr Young was photographed with other members of the Oath Keepers inside the White House with conservative hatchetman Roger Stone.
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