Daniel Rodriguez, 40, admitted to participating in the violent assault of former Metropolitan Police Officer Michael Fanone.
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Fanone was brutally injured, and at one point, lost consciousness. He also suffered a heart attack while trying to fend off the mob of Donald Trump supporters outside the Capitol.
Rodriguez is scheduled to be sentenced in May.
In a taped interview with the FBI following his arrest, Rodriguez admitted he drove a stun gun into the base of Fanone's neck as the officer was trying to fight off rioters who had pulled him down steps on the Capitol's West Front steps.
"What do you want me to tell you? That I tased him? Yes," Rodriguez told investigators. "Am I a f------- piece of s---? Yes."
Rodriguez and another co-defendant charged in the attack on Fanone were set to go to trial later this month. Last September, another rioter who had pleaded guilty to assaulting Fanone was sentenced to more than seven years in prison.
Among the offenses Rodriguez pleaded guilty to Tuesday were conspiracy to obstruct an official proceeding, assaulting a law enforcement officer with a deadly weapon, obstruction and evidence tampering. The offenses collectively carry a potential sentence ranging up to decades in prison.
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An attorney for the Justice Department also said during the plea hearing that the government is reserving its right to potentially seek a sentencing enhancement for Rodriguez based on an argument that his conduct could have amounted to terrorism.
Fanone suffered a heart attack from the assault and later became one of the most outspoken former members of law enforcement calling for accountability for those who incited the mob on Jan. 6. He resigned from D.C. police in late 2021.
In his interview with the FBI, Rodriguez further admitted he broke a window to get into the Capitol and believed that he was following instructions from former President Donald Trump.
"I thought that there was going to be fighting, for some reason, in different cities and I thought that the main fight, the main battle, was going to be in D.C. because Trump called everyone there," Rodriguez said. "I thought we were going to occupy the building. I thought we were going to take a couple days or something."
ABC News contributed to this report.