WASHINGTON — A member of a neo-Nazi group pleaded guilty to Capitol riot charges Thursday, admitting he threw a water bottle at police during the brutal battle at the lower west tunnel and stole a police helmet as a “war trophy” during the Jan. 6 attack.

Richard Zachary Ackerman pleaded guilty to two charges: obstructing law enforcement officers during civil disorder and theft of government property. He had initially faced three additional charges: assault, resist, oppose, impede with a person engaged in performance of official duties; entering and remaining in a restricted building or grounds; and disorderly and disruptive conduct in a restricted building or grounds. Those charges will be dismissed under a plea agreement.

In the court filing Thursday, Ackerman admitted bragging that he stole a SWAT team officer’s helmet, which he labeled a “war trophy.” The FBI said it had received a tip that included a photo of a U.S. Capitol Police helmet that included a “New England 131” sticker on it on Jan. 8, 2021, two days after the attack.

An FBI task force officer said “New England131” referred to the “Nationalist Socialist Club 131” and the “White Defense Force” and described NSC-131 as “a neo-Nazi group with small, autonomous regional chapters in the United States and abroad, whose members see themselves as soldiers at war with a hostile, Jewish-controlled system that is deliberately plotting the extinction of the white race.”

A confidential FBI human source attended a 2021 meeting with members of NSC-131, where a member known online as “Zach Parker” talked about being at the Capitol on Jan. 6 and said he still had the helmet at his residence, according to the FBI task force officer.

Ackerman flew to Germany in July 2021 and was subject to a secondary inspection by U.S. Customs and Border Protection, according to the FBI. Agents who searched his phone said they found conversations in which Ackerman bragged about being at the Capitol. One person he spoke with told him to go home and not to get arrested, saying Ackerman “won’t be able to be a Marine” if he was arrested. (Jan. 6 rioters James Mault and Aiden Bilyard joined the military after they took part in the attack, while numerous other rioters were active-duty members of the military when they took part in the riot.)

In 2022, agents obtained a search warrant for Ackerman’s residence, spoke with a man who identified himself as Ackerman’s father and found a Capitol Police helmet in the fireplace of his basement bedroom, the FBI said.

Ackerman was first arrested in New Hampshire in June and has been on pretrial release ever since. He will remain on release until at least his sentencing before U.S. District Judge Timothy J. Kelly, which is scheduled for July 25.

As NBC News reported, out of more than 1,380 Jan. 6 defendants, just 15 are being held in pretrial detention at the order of a federal judge. All other Jan. 6 rioters behind bars are there because they either admitted in court that they engaged in criminal activity on Jan. 6 or because judges or unanimous juries found them to have committed crimes.

Prosecutors have secured convictions against an estimated 984 defendants so far, and hundreds of additional rioters have been identified by online “sedition hunters” but not yet arrested. Of the roughly 859 defendants who have been sentenced, about 520 have been given at least some period of incarceration, according to the Justice Department, with sentences from a few days behind bars to 22 years in prison.