Ali Alexander, a prominent organizer of Stop the Steal rallies with ties to far-right members of Congress who sought to invalidate the 2020 election results, is cooperating with the House committee investigating the Jan. 6 Capitol riot, pledging to deliver a trove of documents that could shed light on the activities that preceded the attack.

The participation of Mr. Alexander, who is scheduled to be deposed by the panel on Thursday, could provide insight into the nature and extent of the planning by President Donald J. Trump and his Republican allies in Congress for their bid to overturn the election on Jan. 6. It could also help clarify whether and to what degree the prospect of violence was discussed or contemplated before or during the rampage.

Members of the panel said they wanted to dig into Mr. Alexander’s communications with Republican members of Congress and White House officials.

Mr. Alexander, a provocateur who rose in right-wing circles in the chaotic aftermath of the 2020 presidential election, was one of a handful of planners who put together marches and rallies around the country protesting the outcome. The events culminated with the one in Washington on Jan. 6 that brought together throngs of attendees who went on to storm the Capitol.

He attended Mr. Trump’s incendiary speech at the Ellipse near the White House that day, then marched with the crowd toward the Capitol, along with the conspiracy theorist Alex Jones of Infowars. Mr. Alexander arrived, as he put it in his prepared remarks to the panel, “in the early stages of the lawbreaking.”

Mr. Alexander, who has been banned from Twitter for spreading false statements about the election, denies he was to blame for the violence.

“Anyone who suggests I had anything to do with the unlawful activities on Jan. 6 is wrong,” Mr. Alexander wrote in a memo to the committee. “They’re either mistaken or lying.”

Late last month, the House committee issued subpoenas for both Mr. Alexander and Mr. Jones, suggesting that they might have knowledge of how the Stop the Steal rallies on Jan. 6 came together.

“We need to know who organized, planned, paid for and received funds related to those events, as well as what communications organizers had with officials in the White House and Congress,” Representative Bennie Thompson, Democrat of Mississippi and the committee chairman, said at the time.

Mr. Alexander planned to use his deposition on Thursday to proclaim his innocence, telling the select committee that he had “nothing to do with any violence or lawbreaking” that day and accusing other rally organizers of having done little or nothing as the mob stormed the Capitol, according to a copy of his opening statement obtained by The New York Times.

He also accused them of removing from an event program important instructions for where the crowd was to go and partying in an upscale Washington hotel during the violence.

“While I was actively trying to de-escalate events at the Capitol and end the violence and lawlessness, it’s important to note that certain people were nowhere to be found,” Mr. Alexander planned to tell investigators, promising to turn over documents to respond to a congressional subpoena. “Press reports suggest they may have had their feet up drinking donor-funded champagne in a war room in the Willard. I don’t know where they were. But they weren’t working with police trying to de-escalate the chaos like I was.”

It is not clear where Mr. Alexander was or what he was doing during the riot. But in the weeks before the attack, Mr. Alexander repeatedly referred during Stop the Steal events to the possible use of violence to achieve his organization’s goals. He claimed to have been in communication with the White House and members of Congress about events planned to undermine Congress’s official count of the Electoral College results, the committee said.

Mr. Alexander has said that he, along with Representatives Mo Brooks of Alabama, Paul Gosar of Arizona and Andy Biggs of Arizona, all Republicans, set the events of Jan. 6 in motion.

“We four schemed up of putting maximum pressure on Congress while they were voting,” Mr. Alexander said in a since-deleted video posted online, “so that who we couldn’t lobby, we could change the hearts and the minds of Republicans who were in that body, hearing our loud roar from outside.”

Mr. Brooks, who wore body armor onstage on Jan. 6 as he told the crowd to “start taking down names and kicking ass,” and Mr. Biggs, who provided a video message for Mr. Alexander to play at a Dec. 19 rally, have denied coordinating event planning with Mr. Alexander.

Mr. Gosar’s chief of staff called Mr. Alexander “a solid organizer,” but said his office merely promoted Stop the Steal events and was not involved in planning them.

In his opening statement to the committee, Mr. Alexander planned to give a flavor of his personal biography — his mother was Black and lived in public housing; his father, an Arab, disappeared from his life at a young age — and to suggest that he had become a target for those looking to blame the violence of Jan. 6 on someone.

“It is not uncommon in the aftermath of historic chaos and disruption to look for a boogeyman,” his opening statement says. “After all, someone must be held accountable, right?”

Mr. Alexander also intended to describe some of the bitter rivalries that divided the small group of planners that put together large pro-Trump events in Washington last November, December and January. According to the prepared statement, he planned to fault Amy Kremer and her daughter Kylie Kremer, who ran a group called Women for America First that helped to organize the Jan. 6 events.

He said the leaders of the event at the Ellipse removed instructions from the program telling attendees exactly where to go and what to do after the event concluded.

And Mr. Alexander said the event that he scheduled never took place, because the crowd from the Ellipse had already turned into a mob.

“The ‘One Nation Under God’ event that Stop the Steal was a part of did not start the chaos,” he said.

A spokesman representing Women for America First did not immediately respond to a request for comment.

In the past few weeks, Mr. Alexander claims to have spent more than 100 hours searching his archives for “relevant and responsive documentation to this committee’s requests,” according to his statement. He says that he has hired “attorneys and computer consultants to be as responsive as possible.”

Mr. Alexander’s cooperation comes as the committee is moving forward a criminal contempt of Congress referral against a third recalcitrant witness, Mark Meadows, who served as Mr. Trump’s White House chief of staff.

The committee has now interviewed more than 275 witnesses, obtained tens of thousands of documents and is receiving cooperation from some members of former Vice President Mike Pence’s inner circle, including Marc Short, his former chief of staff.

But several high-profile witnesses are stonewalling the panel, in line with a directive from Mr. Trump. The former president is battling in court to block the release of documents requested by the committee that he says are subject to executive privilege, though the Biden administration has refused to assert the claim.