Special counsel Robert S. Mueller III has already interviewed two dozen current and former Trump aides, according to people familiar with the investigation. (Andrew Harnik/AP)

A white sedan whisked a man into the loading dock of a glass and concrete building in a drab office district in Southwest Washington. Security guards quickly waved the vehicle inside, then pushed a button that closed the garage door and shielded the guest’s arrival from public view.

With his stealth morning arrival Thursday, White House Counsel Donald F. McGahn II became the latest in a string of high-level witnesses to enter the secretive nerve center of special counsel Robert S. Mueller III’s investigation into Russian interference in the 2016 election.

Twenty hours later, Mueller and his team emerged into public view to rattle Washington with the dramatic announcement that former national security adviser Michael Flynn would plead guilty to lying to the FBI.

The ensnaring of Flynn, the second former aide to President Trump to cooperate with the inquiry, serves as the latest indication that Mueller’s operation is rapidly pursuing an expansive mission, drilling deeper into Trump’s inner circle.

In the past two months, Mueller and his deputies have received private debriefs from two dozen current and former Trump advisers, each of whom has made the trek to the special counsel’s secure office suite.

The Washington Post's Amber Phillips talks about what likely caused President Trump's former national security adviser, Michael Flynn, to flip on his old boss and cooperate with the Mueller investigation. (Jason Aldag, Amber Phillips/The Washington Post)

Once inside, most witnesses are seated in a windowless conference room where two- and three-person teams of FBI agents and prosecutors rotate in and out, pressing them for answers.

Among the topics that have been of keen interest to investigators: how foreign government officials and their emissaries contacted Trump officials, as well as the actions and interplay of Flynn and Jared Kushner, the president’s son-in-law.

Mueller’s group has also inquired whether Flynn recommended specific foreign meetings to senior aides, including Kushner. Investigators were particularly interested in how certain foreign officials got on Kushner’s calendar and the discussions that Flynn and Kushner had about those encounters, according to people familiar with the questions.

Often listening in is the special counsel himself, a sphinx-like presence who sits quietly along the wall for portions of key interviews.

This picture of Mueller’s operation — drawn from descriptions of witnesses, lawyers and others briefed on the interviews — provides a rare look inside the high-stakes investigation that could implicate Trump’s circle and determine the future of his presidency.

The locked-down nature of the probe has left both the witnesses and the public scrutinizing every move of the special counsel for meaning, without any certainty about the full scope of his investigation.

Trump and his lawyers have expressed confidence that Mueller will swiftly conclude his examination of the White House, perhaps even by the year’s end. Trump’s Democratic opponents hope the investigation will uncover more crimes and ultimately force the president’s removal from office.

Meanwhile, some witnesses who have been interviewed came away with the impression that the probe is unfolding and far from over.

“When they were questioning me, it seemed like they were still trying to get a feel of the basic landscape of the place,” said one witness who was questioned in late October for several hours and, like others, requested anonymity to describe the confidential sessions. “I didn’t get the sense they had anything incriminating on the president. Nor were they anywhere close to done.”

A spokesman for Mueller declined to comment, citing the sensitivity of the ongoing investigation.

White House lawyer Ty Cobb said he believes the probe’s focus on Trump’s White House is wrapping up, noting that all White House staffer interviews will be completed by the end of next week.

“At the end of the interviews, it would be reasonable to expect that it would not take long to bring this to conclusion,” Cobb said. “I commend the Office of Special Counsel for their acknowledged hard work on behalf of the country, to undertake this serious responsibility, and to perform it in an expedited but deliberate, thorough way.”

[Top FBI official assigned to Mueller’s Russia probe said to have been removed after sending anti-Trump texts]

At least two dozen people who traveled in Trump’s orbit in 2016 and 2017 — on the campaign trail, in his transition operation and then in the White House — have been questioned in the past 10 weeks, according to people familiar with the interviews.

The most high profile is Kushner, who met with Mueller’s team in November, as well as former chief of staff Reince Priebus and former press secretary Sean Spicer. Former foreign policy adviser J.D. Gordon has also been interviewed.

White House communications director Hope Hicks was scheduled to sit down with Mueller’s team a few days before Thanksgiving. Mueller’s team has also indicated plans to interview senior associate White House counsel James Burnham and policy adviser Stephen Miller.

McGahn, who was interviewed by Mueller’s prosecutors for a full day Thursday, was scheduled to return Friday to complete his interview. However, the special counsel postponed the session as a courtesy to allow McGahn to help the White House manage the response to Flynn’s plea, a person familiar with the interview said.

Cobb declined to say which White House aides remain to be interviewed.

Several people who worked shoulder to shoulder with Flynn have also been interviewed by Mueller’s operation. That includes retired Gen. Keith Kellogg, the chief of staff to the National Security Council, as well as several people who worked with Flynn Intel Group, a now-shuttered private consulting firm.

During the transition, Kushner and Flynn met with the Russian ambassador to the United States, Sergey Kislyak. At the early December meeting, Kushner suggested establishing a secure communications line between Trump officials and the Kremlin at a Russian diplomatic facility, according to U.S. officials who reviewed intelligence reports describing Kislyak’s account.

Kushner has said that Kislyak sought the secure line as a way for Russian generals to communicate to the incoming administration about U.S. policy on Syria.

Trump’s son-in-law has also been identified by people familiar with his role as the “very senior member” of the transition team who directed Flynn in December to reach out to Kislyak and lobby him about a U.N. resolution on Israeli settlements, according to new court filings.

The volume of questions about Kushner in their interviews surprised some witnesses.

“I remember specifically being asked about Jared a number of times,” said one witness.

Another witness said agents and prosecutors repeatedly asked him about Trump’s decision-making during the May weekend he decided to fire FBI Director James B. Comey. Prosecutors inquired whether Kushner had pushed the president to jettison Comey, according to two people familiar with the interview.

Kushner attorney Abbe Lowell declined to comment on what the president’s son-in-law discussed at his November session with Mueller. “Mr. Kushner has voluntarily cooperated with all relevant inquiries and will continue to do so,” he said.

Two administration officials said that it would be natural for investigators to ask a lot of questions about Kushner, whom Trump put in charge of communicating with foreign officials, adding that such inquiries do not indicate he is a target.

The special counsel has continued to make ongoing requests for records from associates of the Trump campaign, according to two people familiar with the requests. The campaign associates aren’t expected to finish producing these documents by the end of the year. Mueller’s team is also newly scrutinizing an Alexandria-based office and advisers who worked there on foreign policy for the campaign.

In the past several weeks, Mueller’s operation has reached out to new witnesses in Trump’s circle, telling them they may be asked to come in for an interview. One person who was recently contacted said it is hard to find a lawyer available for advice on how to interact with the special counsel because so many Trump aides have already hired attorneys.

“It was kind of a pain,” the person said. “It’s hard to find a lawyer who wasn’t already conflicted out.”

People who have gone before Mueller’s team describe polite but detailed and intense grillings that at times have lasted all day and involved more than a dozen investigators. Spicer, for example, was in the office from about 10 a.m. until 6 p.m. for his fall session. Mueller’s team has recommended nearby lunch spots, but many witnesses have food brought in for fear of being spotted if they go outside.

Mueller has attended some interviews, introducing himself to witnesses when he enters and then sitting along the wall. Sometimes he is joined by his deputy, longtime friend and law partner James Quarles, a former Watergate prosecutor who is the main point of contact for the White House.

Investigators bring large binders filled with emails and documents into the interview room. One witness described the barrage of questions that followed each time an agent passed them a copy of an email they had been copied on: “Do you remember this email? How does the White House work? How does the transition work? Who was taking the lead on foreign contacts? How did that work? Who was involved in this decision? Who was there that weekend?”

[Here are the people investigating Russian meddling in the 2016 election]

Some witnesses were introduced to so many federal agents and lawyers that they later lamented that they had largely forgotten many of their names by the time one team left the room and a new team entered.

“They say, ‘Hey, we’re not trying to be rude, but people are going to come in and out a lot,’ ” one witness explained about the teams. “They kind of cycle in and out of the room.”

One contingent of investigators is focused on whether Trump tried to obstruct justice and head off the investigation into Russian meddling by firing Comey in May. Prosecutors Brandon Van Grack and Jeannie Rhee have been involved in matters related to Flynn.

Yet another team is led by the former head of the Justice Department’s fraud prosecutions, Andrew Weissman, and foreign bribery expert Greg Andres. Those investigators queried lobbyists from some of the most powerful lobby shops in town about their interactions with former Trump campaign chairman Paul Manafort and campaign adviser Rick Gates.

Mueller’s team charged Manafort and Gates last month with engaging in a conspiracy to hide millions of dollars in foreign accounts and secretly creating an elaborate cover story to conceal their lobbying work for a former Ukrainian president and his pro-Russian political party. Both have pleaded not guilty.

Lawyers familiar with prosecutors’ questions about Manafort said they expect several more charges to come from this portion of the case.

People familiar with the Mueller team said they convey a sense of calm that is unsettling.

“These guys are confident, impressive, pretty friendly — joking a little, even,” one lawyer said. When prosecutors strike that kind of tone, he said, defense lawyers tend to think: “Uh oh, my guy is in a heap of trouble.”

Rosalind S. Helderman contributed to this report.