WASHINGTON — A U.S. Capitol Police officer was arrested Friday on charges that he obstructed justice by telling a man who had entered the Capitol illegally during the Jan. 6 riot to delete evidence of his actions that day from his social media accounts.

Michael A. Riley, 50, a member of the agency’s K-9 unit with more than 25 years on the force, is the first officer charged with a crime in connection with the Jan. 6 attack on the Capitol, when scores of his fellow officers were beaten, bloodied and injured by a pro-Trump mob spurred on by the lie of widespread election fraud.

He was released pending an Oct. 26 hearing.

According to an indictment from a federal grand jury in Washington, on Jan. 7 Officer Riley contacted an acquaintance who had posted images on Facebook of himself inside the Capitol during the attack to encourage him to take down the evidence that he had been in the building. Officer Riley did not know the man personally, the indictment said, but had recently become acquainted with him through an online group for fishing enthusiasts.

“I’m a Capitol Police officer who agrees with your political stance,” the officer wrote to the man, according the indictment. “Take down the part about being in the building they are currently investigating and everyone who was in the building is going to charged. Just looking out!”

Officer Riley and the man then exchanged dozens of messages.

“I’m glad you got out of there unscathed,” Officer Riley wrote at one point. “We had over 50 officers hurt, some pretty bad.”

Officer Riley responded to reports of an explosive device near the Capitol on Jan. 6, but was not defending the building when the mob stormed in, disrupting Congress’s official count of electoral votes to confirm President Biden’s victory.

On Jan. 20, the unidentified man turned himself in to police and told them he had been talking with Officer Riley, then warned the officer that federal law enforcement officials were aware they had been communicating.

“The F.B.I. was very curious that I had been speaking to you if they haven’t already asked you about me they are gonna,” the man wrote to Officer Riley, according to the indictment. “They took my phone and downloaded everything.”

After receiving that message, Officer Riley deleted all his Facebook messages with the man, and the next day, sent him a final Facebook message, according to the indictment.

“Another mutual friend was talking about you last night. I tried to defend you but then he showed me a video of you in the Capitol smoking weed and acting like a moron,” he wrote. “I have to say, I was shocked and dumbfounded, since your story of getting pushed in the building with no other choice now seems not only false but is a complete lie. I feel like a moron for believing you.”

Officer Riley was charged with two counts of obstruction of justice, each of which carries a maximum penalty of 20 years in person.

In a statement, Chief J. Thomas Manger of the Capitol Police called the allegations “very serious,” and said the department’s Office of Professional Responsibility would open an administrative investigation into the officer’s conduct.

“The department was notified about this investigation several weeks ago,” he said in a statement. “Upon his arrest, the officer was placed on administrative leave pending the completion of the case.”

The charges against Officer Riley come after an internal Capitol Police investigation recommended that six other officers be disciplined based on their actions during the riot. Three officers were singled out for unbecoming conduct, one officer for failure to comply with directives, one officer for improper remarks and one officer for improper dissemination of information, the Capitol Police said in a statement.

None of those officers were named or charged with a crime.

Even as the majority of the police force has grappled with the trauma of the attack, videos widely circulating on social media appeared to show some officers treating the rioters sympathetically or doing little to stop them from entering the complex.

Officer Brian D. Sicknick of the Capitol Police died of a stroke in the hospital after fending off the mob, and at least 73 officers were hurt that day after being assaulted with flagpoles, fire extinguishers and hockey sticks, injuries that ranged from bruises to concussions and burns.

Two officers who clashed with the mob later took their own lives.