WASHINGTON — Behind closed doors inside a nondescript office building at the foot of Capitol Hill on a recent chilly morning, the House inquiry into the Jan. 6 attack on the Capitol was in full swing.

As congressional staff aides shuffled through the halls going about their normal business, investigators quietly pulled shades over the windows in conference rooms on several different floors and posted “Do Not Disturb” signs.

In one such room sat Ali Alexander, a prominent organizer of the Stop the Steal rallies with ties to far-right members of Congress who worked to help Donald J. Trump invalidate his 2020 election loss.

A floor below was Kash Patel, a former Pentagon chief of staff involved in discussions about Capitol security. He had been in constant contact with Mr. Trump’s former chief of staff, Mark Meadows, on Jan. 6.

Facing questions elsewhere in the building were John Eastman, a lawyer who plotted with Mr. Trump and his allies to overturn the 2020 election results; and Christopher Krebs, the Trump administration’s most senior cybersecurity official, who was fired after systematically dismantling Mr. Trump’s false declarations that the presidency had been stolen from him.

The committee scrutinizing the pro-Trump mob attack has conducted much of its inquiry in private, drawing public attention mostly for the legal fights it is waging over access to evidence from Mr. Trump and some of his top lieutenants. But from a warren of offices in the O’Neill House Office Building in Southwest Washington, a few dozen investigators and members of Congress have ramped up a sprawling and elaborate investigation into the worst American attack on democracy in centuries.

In recent weeks, with the anniversary of the riot looming on Thursday, the panel has redoubled its efforts in the face of mounting resistance from the former president. It is rushing to make as much progress as possible before January 2023. Republicans are favored to regain control of the House this fall, and if they do, that is when they would take power and almost certainly dissolve the inquiry.

“We worked on Christmas and on New Year’s Day,” said Representative Bennie Thompson, Democrat of Mississippi and the chairman of the committee. “The window for getting the job done requires weekends and holidays too. There’s a really firm commitment on the part of the staff to get it done.”

Working in color-coded teams, investigators have interviewed more than 300 witnesses, from White House officials close to Mr. Trump to the rioters themselves, and are sorting through more than 35,000 documents. During its first three months, from July through September, the committee had fewer than 30 staff members and spent about $418,000, according to the latest documents filed with the House. Since then, the panel has increased its staff to about 40 and is looking to hire more investigators.

Soon, the inquiry will enter a new phase, with plans to hold a series of public hearings in early spring to lay out some of its findings. Those will feature, among other topics, state election officials testifying to the security and accuracy of the 2020 election. A final report will be issued, “obviously before the November elections,” Mr. Thompson said.

For now, the O’Neill building is the main hub of activity, where, depending on the day, the political operative Roger J. Stone Jr., a longtime adviser to Mr. Trump, might appear outside flashing his signature Nixon-style “V for victory” sign to a sea of news cameras; or a lawyer for a Jan. 6 rally planner might arrive promoting a “treasure trove” of documents he says will leave senior Trump allies “quivering in their boots.” Reporters often dart up and down hallways trying to catch up to the various witnesses leaving the interview rooms.

Inside, investigators and members of the nine-person committee are questioning witnesses, with the lawmakers — juggling busy schedules of floor votes and other congressional hearings — often bouncing between the interviews on a direct TV feed.

“We are participating in the depositions and interviews regularly, and these are quite lengthy,” said Representative Elaine Luria, Democrat of Virginia and a member of the committee. “Even with other work that we have to do throughout the day, members are joining regularly to ask questions about specific areas.”

The so-called green team is following the money trail connected to Mr. Trump’s efforts to promote the baseless assertion that he was the rightful winner of the election, including whether any groups defrauded contributors with false statements about widespread election fraud.

The gold team is scrutinizing any plans Mr. Trump made with members of Congress to try to overturn the election and his pressure campaign on local, state and Justice Department officials to try to keep himself in power.

Domestic violent extremist groups, such as the QAnon movement and the militia groups, the Oath Keepers and the Proud Boys, are the focus of the purple team. A fourth, the red team, is digging into the Jan. 6 rally planners and the Stop the Steal movement.

The committee is led by Mr. Thompson and Representative Liz Cheney, Republican of Wyoming, who serves as vice chairwoman. Its top two investigators — both former U.S. attorneys — also come from different parties.

Timothy J. Heaphy, whom President Barack Obama named U.S. attorney for the Western District of Virginia, is the Jan. 6 committee’s chief investigative counsel; and John Wood, whom President George W. Bush hired as U.S. attorney for the Western District of Missouri, is the committee’s senior investigative counsel.

Mr. Wood, an ally of Ms. Cheney, is closely supervising the team focused on Mr. Trump’s direct involvement.

One witness recently interviewed by the committee said arriving at the O’Neill building, a gleaming glass-encased behemoth, was like entering the British intelligence agency’s headquarters, with its modern lines and sterile feel. A congressional staffer escorted him up an elevator to a room with a U-shaped table and a large television on the wall. The TV had a live remote feed through which members of the committee could watch and listen.

The witness said that before the deposition questioning began, he had been presented with a large binder full of evidence that investigators had collected on him. The lawyers conducting the inquiry were often “adversarial and hostile” in tone, he said, and were interested in the most minute details, even the moods and emotions of the people they were asking about.

The committee appeared to be interested in a few key areas: any connections between the violence that took place on Jan. 6 and the group of political activists who planned the pro-Trump rallies that preceded it; any connections between the rally organizers and people in Mr. Trump’s orbit, including members of Congress; and any role that White House aides, Trump campaign officials and members of Mr. Trump’s family may have played in the putting the rallies together.

“These are professionals — these are very skilled individuals who understand how to question witnesses to get to the truth,” Ms. Cheney said of the panel’s investigators. “It’s not all antagonistic. We’ve had the benefit of scores of people coming forward to say, ‘We’d like to help. We have suggestions for other people you should talk to.’”

As they investigate, committee members are looking into whether a range of crimes were committed, including two in particular: whether there was wire fraud by Republicans who raised millions of dollars off assertions that the election was stolen, despite knowing the claims were not true; and whether Mr. Trump and his allies obstructed Congress by trying to stop its formal count of electoral votes.

Recently, the committee has begun issuing subpoenas for bank records. Mr. Thompson said some of the financing of the rallies and advertisements that spread the misinformation about a stolen election could have run afoul of various federal laws.

“We have several subpoenas in the queue,” Mr. Thompson said in an interview.

The committee also is trying to obtain unreleased takes of Mr. Trump’s Jan. 6 video, in which he told rioters — hours into the violence — to go home, but also insisted that the election was stolen and fraudulent. “We love you. You’re very special,” Mr. Trump said to the rioters in the clip that was ultimately released.

Witnesses have told investigators that Mr. Trump recorded other takes of the video, in which he did not tell the crowd to leave, that were never shown to the public. Mr. Thompson said these takes could shed light on his mindset at the time.

“We need to know as a committee, why did it take so long, as this riot was going on at the Capitol, for you to say, ‘Stop?’ ” Mr. Thompson said of Mr. Trump.

The panel’s investigators have not been slowed by the partisan infighting that sometimes takes place among committee staff on other congressional panels. Because Representative Kevin McCarthy of California, the minority leader, decided Republicans would not participate in the panel, there are no staff members in the interviews running interference for Mr. Trump.

“We’re moving as swiftly as I think any congressional committee ever has,” said Representative Adam B. Schiff, Democrat of California, who led the first impeachment proceeding against Mr. Trump.

But the panel is still facing a litany of lawsuits filed by Mr. Trump and his allies to try to block or slow down the committee’s subpoenas and document demands. Though the committee has interviewed hundreds of witnesses, some of the most crucial have yet to cooperate, Mr. Schiff said.

“Some witnesses are far more important than others, and I think that some really important witnesses are attempting to deprive the committee and American people of what they know,” Mr. Schiff said. “There’s still some very significant witnesses and very significant documents we haven’t obtained.”