Ali Alexander, the prominent pro-Trump activist, will cooperate with the justice department investigation into the Capitol attack, making him the first high-profile political figure to agree to assist the government’s criminal inquiry into the events of January 6.

The move is likely to give initial momentum to the newly expanded justice department investigation running in parallel to the House select committee investigation examining Donald Trump and the Capitol attack.

An attorney for Alexander – the organizer of the “Stop the Steal” movement – told the Guardian that he had agreed to cooperate with the justice department since he was left with no choice after being issued a grand jury subpoena, and had been informed by the prosecutor that he was not currently a target of the investigation.

The news of his cooperation was earlier reported by the New York Times.

In a lengthy statement through his attorney, Alexander denounced the process as “hostile” but indicated he would comply with the grand jury subpoena asking about the “Women for America First” group and the “Save America March” events that immediately preceded the Capitol attack.

“I did nothing wrong and I am not in possession of any evidence that anyone else had plans to commit unlawful acts,” Alexander said in the statement. He has also denounced anyone who took part in or planned violence on 6 January 2021.

Alexander said he did not think he could provide prosecutors with anything useful for the inquiry, noting he had not financed the equipment used for the Save America rally on the Ellipse near the White House and had not discussed the security for the event with the Trump White House.

The statement added that he had not coordinated any movements with the Proud Boys militia group and he had only accepted an offer from the Oath Keepers militia group to act as security for a separate event he had planned near the Capitol, which ultimately did not take place.

It was not clear what assistance Alexander might furnish. But he was deeply involved in efforts to invalidate the results of the 2020 election and had contacts with members of Congress and, according to the House select committee, White House officials.

That is now of interest to the justice department, which recently expanded the scope of its January 6 inquiry to include Trump’s push to return himself to office, after spending months focused purely on the rioters that stormed the Capitol.

A spokesperson for the justice department declined to comment.

The subpoena to Alexander from the grand jury empaneled by federal prosecutors suggests the justice department investigation could go beyond that of the select committee, to which he testified voluntarily for about eight hours last December.

It also indicates that the criminal inquiry could reach Trump’s inner circle, with the subpoena demanding information about members of the legislative and executive branches who were involved in efforts to obstruct the certification of Joe Biden’s election win.