The man who was photographed inside the US Capitol during the 6 January insurrection shirtless, wearing a horned headdress and furs, and heavily tattooed, pleaded guilty on Friday to obstructing an official proceeding when he took part in the assault by extremist supporters of then president Donald Trump.

Jacob Chansley, of Phoenix, Arizona, who at the time was known to some as the so-called “QAnon Shaman”, had been held without bond since his arrest shortly after the riot.

While in detention, Chansley underwent mental examinations and was diagnosed by prison officials with transient schizophrenia, bipolar disorder, depression and anxiety.

Nearly 600 people have been arrested over the attack on the Capitol where Congress was meeting to certify Joe Biden’s November victory over Trump.

Earlier, Trump had given a fiery speech falsely claiming his defeat was the result of widespread election fraud, despite officials at local, state and national level calling it the most secure presidential ballot in US history.

While the charge against Chansley carries both a maximum 20-year prison term and a fine of up to $250,000, prosecutor Kimberly Paschall indicated the maximum sentence the government was likely to request would be much shorter.

Chansley had been a supporter of the QAnon conspiracy theory that casts Trump as a savior figure and elite Democrats as a cabal of satanist pedophiles and cannibals.

After he turned himself in following the insurrection, he explained to the FBI that he was part of the mob who broke into the House chamber as a “patriot”, at the request of the president.

He told investigators: “This is not America. They’re shooting at us. They’re supposed to shoot BLM [Black Lives Matter supporters], but they’re shooting the patriots.”

In an earlier news release Albert Watkins, Chansley’s lawyer, asserted that Chansley “has repudiated the ‘Q’ previously assigned to him as part of a nickname and requests future references to him be devoid of use of the letter ‘Q’”.

In the months before Friday’s hearing, senior US district judge Royce Lamberth rejected multiple requests from Chansley for possible pre-trial release.

On Friday, Watkins asked the judge to allow Chansley to be released from prison pending a sentencing hearing, scheduled for 17 November. The judge said he would consider this request.

Watkins noted that prosecutors had acknowledged Chansley was “not a planner or organizer” of the riot.