WASHINGTON (Reuters) - When members of Special Counsel Robert Mueller’s team investigating Russia’s role in the 2016 U.S. election have arrived for work each day, they have placed their mobile phones in a locker outside of their office suite before entering.

Operating in secrecy in a nondescript glass-and-concrete office, the team of prosecutors and investigators since May 2017 has unearthed secrets that have led to bombshell charges against several of President Donald Trump’s aides, including his former national security adviser, campaign chairman and personal lawyer, who have pleaded guilty or been convicted by a jury.

To protect those secrets from prying ears, the whole of the office suite in southwest Washington has been designated a Sensitive Compartmented Information Facility (SCIF), U.S. spy speak for an area that has restrictions to ensure secret information stays secure.

One common restriction in SCIFs is to keep out smartphones and other electronic devices, which can be turned into covert listening devices or spy cameras. Visitors also have been required to turn these over before entering.

The restrictions, while not surprising given the team was investigating whether a hostile foreign power tried to help Trump win the 2016 election and whether his campaign conspired in the effort, have not been previously reported.

Mueller on Friday sent his report on his investigation to U.S. Attorney General William Barr, setting off a clamor from lawmakers in both Democratic and Republican parties for the document’s public release.

Accounts of witnesses interviewed by the special counsel’s team, their lawyers and others familiar with the investigation reveal the lengths to which Mueller, a former FBI director, has gone to ensure his high-profile probe safeguarded its secrets.

In a city known for its leaks, Mueller has pulled off a rare feat. He has kept a tight lid on both his office and the evidence he was amassing in his highly sensitive investigation that has cast a cloud over Trump’s presidency. And he did it even as Trump relentlessly criticized him, calling the probe a “witch hunt” and the special counsel’s team “thugs.”

THE ADVISER AND THE DODGE CHARGER

When former Trump campaign adviser Michael Caputo agreed to an interview with Mueller in May 2018, he was told he would be picked up at the hotel where he was staying in Washington. On the lookout for a black government SUV, Caputo and his lawyer were surprised when an FBI agent drove up in his personal car, a white Dodge Charger.

“Then he drove us 15 blocks to their location and we went in through the garage so that nobody would see,” Caputo said in an interview.

Caputo was questioned about former Trump campaign chairman Paul Manafort, Manafort’s aide Rick Gates and longtime Trump adviser Roger Stone. When the interview was over, Mueller’s team told him they would take him back to his hotel. Caputo said Mueller’s team was not happy with what he said next.

“I said I’m meeting a TV crew downstairs so I won’t need a ride,” Caputo said. “They weren’t upset that I was talking to the media, they were disturbed that I was doing it in (front of) the office.”

“They were concerned ... that would put their agents and attorneys at risk,” Caputo said, adding that he agreed to meet the news crew at a different location nearby.

Former Trump campaign adviser Sam Nunberg said an FBI agent picked him up at the train station to take him to the office.

“You put your phone and any electronic devices and leave them in a compartment out front,” Nunberg added. “It was a very plain office.”

Nunberg said he went into a conference room with three tables, and prosecutor Aaron Zelinsky, a member of Mueller’s team, came in with three FBI agents, one female and two males.

The office’s location was not publicly revealed but was discovered by journalists. Still, it has not been widely publicized. Mueller’s team has asked media outlets not to publish the exact location for security purposes.

“We are working in a secure location in Southwest DC,” Peter Carr, a spokesman for Mueller, has said.

STAYING OUT OF THE NEWS

“In a town where everybody and their mother is trying to get on the front page, Bob Mueller was always trying to stay out of the news,” said Mark Corallo, a former Justice Department spokesman. “He wanted to be judged on actions, not press conferences.”

FILE PHOTO: FBI Director Robert Mueller listens at the U.S. Senate Judiciary Committee at an oversight hearing about the Federal Bureau of Investigation on Capitol Hill in Washington, June 19, 2013. REUTERS/Larry Downing/File Photo

Corallo, who was briefly a spokesman for Trump’s legal team, was interviewed by Mueller’s team in February 2018.

Corallo and other witnesses summoned for interviews by Mueller’s team said they were picked up from their lawyers’ offices and taken to a secure parking garage in the building in southwest Washington.

The team’s office suite was anonymous with no plaque on the door to identify its occupants, said Washington lawyer A. Joseph Jay, who represented a witness he declined to identify.

More than once, Jay recalled, members of Mueller’s team expressed their commitment to confidentiality. “They made it clear on a number of occasions, ‘We don’t leak. You don’t have to worry about that with us.’”

“By keeping to their code of silence, they were professionals,” Jay said. “They weren’t reacting to the spin. They were doing their jobs. They spoke through a number of indictments. They spoke through a number of sentencing memos.”

Mueller has remained silent throughout the investigation and his office has issued only one statement. In that statement, issued this past January, spokesman Carr labeled as “not accurate” a BuzzFeed News account describing evidence collected by the special counsel that allegedly showed that Trump had directed his former lawyer Michael Cohen to lie to Congress about a Moscow real estate deal. BuzzFeed has stood by its story.

Trump’s lawyer Rudy Giuliani, himself a former federal prosecutor, also remarked on Mueller staying out of sight.

“Whenever we talk to them, they say, ‘We’ll take it to Bob.’ He’s like the Wizard of Oz,” Giuliani said.

Giuliani said although he was suspicious of leaks to the news media, he acknowledged he knew of none for sure from the special counsel’s team and that nothing he told Mueller’s office was leaked.

“Mueller doesn’t talk to us. I don’t know why he’d talk to the press,” the former New York mayor added.

Joseph Campbell, a former assistant director of the FBI’s Criminal Investigative Division who worked at the agency when Mueller headed it, said the special counsel knows how to handle sensitive investigations and ignores the attacks on him.

“He went through 12 years starting with 9/11 of extremely critical and sensitive investigations around the world,” said Campbell, referring to the 2001 attacks on the United States. “This is right in his wheelhouse.”

“He is not affected by external criticism or speculation,” Campbell added.

Robert Litt, former general counsel for the Office of the Director of National Intelligence, said any leaks about the investigation appeared to have come from witnesses or their lawyers.

“There’s nothing he can do about that,” Litt said, referring to Mueller.

Litt said Mueller, the 74-year-old former U.S. Marine Corps officer and architect of the modern FBI, probably “cares little about the public perception of him.”

“He cares,” Litt said, “about doing the job right.”