In a secure location in southwest Washington, D.C., with access to a Sensitive Compartmented Information Facility for classified material, 16 of the country’s top lawyers have passed the last several months working on an investigation that will likely be as consequential as it is secretive.

The following details—gleaned from conversations with people familiar with President Donald Trump’s legal team, as well as intelligence experts and friends of the people working for special counsel Robert Mueller—help explain the broad range of legal and counterintelligence experts he’s assembled. Mueller has essentially built his own miniature Justice Department.

Andrew Weissmann

Weissmann has spent most of his career in the Justice Department—first in the Eastern District of New York, and now at Main Justice. He’s on detail from his position overseeing fraud prosecutions to work with Mueller.

It isn’t their first tour of duty together. Weissmann was Mueller’s general counsel at the FBI for years.

A former FBI official who worked with him there told The Daily Beast that unlike many government attorneys, Weissmann rarely equivocated or dilly-dallied about decisions.

“He was not a paper tiger,” the former official said.

The former official said Weissmann argued doggedly for the FBI’s positions when officials there disagreed with the legal views of attorneys at DOJ headquarters—and was sometimes willing to raise his voice and use obscenities.

“This isn’t gonna fuckin’ stand!” Weissmann yelled at one meeting where FBI officials discussed their differences with the Justice Department, according to that source.

It’s a trait that won him fans at the FBI, and countless foes among criminal defense lawyers. Weissmann generated enormous anger for the hardball tactics he used when he ran the Enron probe—especially his prosecution of the accounting firm Arthur Andersen, which resulted in more than 20,000 people losing their jobs and zero convictions. One prominent white collar defense attorney vowed that Weissmann would never work in private practice because he was so despised over the Andersen case. Despite that, Weissmann made a pit stop at the private firm Jenner & Block for a few years before returning to the FBI.

James Quarles

Quarles is part of the old guard of Washington lawyers and worked on the Watergate prosecution. Besides Mueller himself, Quarles seems to deal with Trump’s legal team more than just about anybody else on the probe.

“Ty [Cobb, one of the president’s lawyers] and I have had excellent relations with [Quarles] and Bob [Mueller], and we are very much appreciative,” said John Dowd, one of the president’s attorneys.

Along with Weissmann, Quarles is one of the most senior people on Mueller’s team. A person familiar with Mueller’s management style said it’s safe to assume Weissmann and Quarles have managerial roles on the probe.

Quarles was a partner at WilmerHale—the predominantly Democratic law firm where Mueller worked before becoming the special counsel—along with a host of other attorneys involved in the probe.

Those include Jamie Gorelick, who was second in command at the Justice Department under Janet Reno and who has represented Jared Kushner on issues related to his security clearance; and Reg Brown, also a partner at the firm, who represented Paul Manafort until about two weeks ago. (Multiple sources told The Daily Beast that Manafort is facing financial strain because of legal costs.)

Aaron Zebley

Zebley is a Mueller whisperer. He was Mueller’s chief of staff at the FBI, often acting as a go-between for Mueller and the bureau’s senior officials, according to Ron Hosko, formerly an assistant FBI director. Mueller mentored Zebley and guided him through the bureau, according to a former DOJ official.

Zebley seems to have a pretty good poker face.

“You could you be giving him your view and he could be thinking, ‘This guy’s a complete idiot’ or ‘This information is completely misshaped!’ and you’d never know,” said a former FBI official who worked with him.

Zebley accompanied Mueller when he briefed the Senate Judiciary Committee on his investigation, according to a source familiar with the meeting.

Jeannie Rhee

Besides Weissmann, Rhee is the attorney whose presence on Mueller’s team has most irked the president’s allies. She previously represented the Clinton Foundation and was an official in the Justice Department’s prestigious Office of Legal Counsel (OLC) during the Obama administration.

A person familiar with the president’s legal team said its representatives have tried to communicate to the special counsel that they worry Rhee’s presence on the team could result in it moving in a partisan direction.

People who know Rhee say that’s laughable. John Bies, who worked alongside her in OLC, said Rhee felt deep personal responsibility for the work of the office.

“She was anxious and had a real sense of responsibility about getting it right,” he told The Daily Beast.

Rhee was also a federal prosecutor in Washington, D.C., where she worked on the prosecution of teachers’ union officials who embezzled millions of dollars to buy tickets to Wizards games and fur coats, according to The Washington Post. And though conservative media figures have criticized Rhee for past contributions to Democrats, she supported the confirmation of Republican Rachel Brand as associate attorney general.

Michael Dreeben

A longtime Washington attorney told The Daily Beast it’s unthinkable that Mueller would have executed the search warrant to raid Manafort’s house without the sign-off of Michael Dreeben.

On the team investigating Russian interference, Dreeben’s legend is second only to that of Mueller’s. Dreeben has spent years in the solicitor general’s office of the Justice Department and has argued before the Supreme Court more than 100 times.

Numerous Washington lawyers said he knows more about U.S. criminal law than anyone else on the planet. One attorney described him as “a demigod of the legal world, respected and feared by everyone in the realm of criminal law.”

Peter Vincent, a former senior DHS official, said Dreeben is an “absolute superstar.” Harold Koh, the top lawyer at the State Department under President Barack Obama, called Dreeben a “brilliant, brilliant lawyer.”

“He’s extremely rational, like Mr. Spock,” Koh added. “He’s not a joker.”

Bies, who has also worked with Dreeben, said the Star Trek comparison was apt “only if you recognize that Dr. Spock was half human, and has emotions in addition to rationality.”

Andrew Goldstein

Goldstein is one of a handful of New Yorkers who headed to D.C. to work on the probe. He’s on detail from his post as head of the Southern District of New York’s public corruption unit. Before taking that job—where he prosecuted New York Assembly Speaker Sheldon Silver and helped dismantle the Liberty Reserve criminal enterprise, which laundered hundreds of millions of dollars using online currency—he was a staff writer for Time magazine, where he covered the Columbine shooting.

Goldstein is the son of Jonathan Goldstein, who was the United States attorney for the District of New Jersey. President Richard Nixon nominated him for that post in 1974.

Elkan Abramowitz, a criminal defense attorney who has practiced in New York for years and has dealt with Andrew Goldstein on legal matters, said he’s widely respected.

“What really is important about him is his judgment,” Abramowitz told The Daily Beast. “He’s very temperate and solid. I would trust his judgment. For example, if he were to conclude that there was insufficient evidence, his judgment could be relied on. If he were to conclude otherwise, his judgment also could be relied on.”

Elizabeth Prelogar

Before heading to the firm Hogan Lovells and then to the solicitor general’s office, Elizabeth Prelogar was a Fulbright scholar in Russia (and speaks Russian). Neal Katyal, who worked with Prelogar and Dreeben as acting solicitor general during the Obama administration, said she was “perhaps the best young lawyer with whom I have ever worked.”

“If I were hand-picking a team of the very best lawyers in the nation, regardless of whatever the issues in a case may be, both of them would be at the top of the list,” he added, “and I know that sentiment is shared by both Republican and Democratic lawyers alike.”

Prelogar is widely viewed as a rising star in the Justice Department.

Brandon Van Grack

Brandon Van Grack is referred to by friends as “BVG.” Josh Geltzer, who heads Georgetown Law’s Institute for Constitutional Advocacy and Protection, worked down the hall from Van Grack when they were both in the Justice Department’s National Security Division.

“It would absolutely make sense that a small team like this would want him at their core because of how impossible it is not to get along with him,” Geltzer said.

Van Grack prosecuted counter-espionage cases and is on loan to the probe from the U.S. attorney’s office for the Eastern District of Virginia, where he is on the national security and international crime unit.

Van Grack has prosecuted a host of crimes that seem to provide extremely relevant experience for his work with Mueller. He’s gone after a member of the Assad-aligned Syrian Electronic Army, helped lock up an Iranian national who tried to smuggle sophisticated technology out of the U.S., and helped successfully prosecute a Michigander who tried to spy for China.

His biggest claim to fame, though—and “fame” may be too strong a word here—is his work prosecuting Ardit Ferizi, a hacker who shared a kill list with ISIS. That was the first time the Justice Department convicted a hacker for providing material support to a terrorist organization.

Rush Atkinson

Like Van Grack, Atkinson has worked in the Eastern District of Virginia on espionage cases and in the DOJ’s National Security Division. He’s on detail to the special counsel from the fraud section of the DOJ’s criminal division, where he worked under Weissmann.

Zainab Ahmad

Of the younger lawyers on Mueller’s team, Ahmad has by far the highest profile. The New Yorker profiled her earlier this year because she has successfully prosecuted 13 terrorism suspects, according to the magazine, and has yet to lose in court.

Aaron Zelinsky

Zelinsky, who went to Yale for undergrad and law school, clerked for Judge Thomas Griffith, a George W. Bush appointee. He also worked under Rod Rosenstein when he was U.S. attorney for Maryland—two GOP-friendly résumé lines that critics of the Mueller probe never mention.

Zelinsky also worked under Koh at the State Department during the Obama administration, where he helped handle hostage negotiations. When American journalist Clare Gillis was held hostage in Libya, Koh said Zelinsky spoke with her family every night.

“The guy was mid- to late-20s, talking to a family that doesn’t know whether their daughter is alive or dead, and are eager for scraps of info,” Koh said. “And he showed tremendous discretion. He never over-promised.”

Koh said Zelinsky also had impressive foresight. At one point, the State Department determined Gillis’s captors were moving her toward Tripoli.

“Aaron comes to me and says, ‘I think we need to call NATO HQ and tell them not to bomb that road,’” Koh said.

Gillis was ultimately freed, along with fellow hostage James Foley. Foley was later taken captive in Syria and beheaded in 2014 by ISIS fighters.

Adam Jed

Jed is one of the only people on Mueller’s team who has never worked as a prosecutor. The Harvard Law graduate has held several posts in the Justice Department, most recently handling appellate litigation in the Civil Division.

“He’s a very smart careful appellate lawyer,” said Bies. “The fact that him and the other solicitor general’s office people were brought in shows Mueller’s playing the long game and thinking carefully about where things will go—not just in the investigation, but down the road when they have to litigate issues in the courts.”

One attorney who practices federal criminal defense noted that Jed has experience handling asset forfeiture, which could be useful if the probe deals with property purchased using criminal proceeds.

Greg Andres

Like Weissmann and Ahmad, Andres worked in the Eastern District of New York U.S. attorney’s office—where Judge Beryl Howell, who is overseeing Mueller’s D.C. grand jury, and former attorney general Loretta Lynch were also prosecutors. During Andres’ time in Brooklyn, he worked on organized crime cases, just like Weissmann.

Andres’ wife, Judge Ronnie Abrams, recused herself from two cases involving the Trump family because of her husband’s work.

Andres is one of the most celebrated trial lawyers currently practicing law. He prosecuted mafia figures and white collar criminals before going into private practice.

In an interview with Law360 published in May 2016, Andres said trial lawyers should always project confidence.

“Be confident, straightforward and well prepared,” he said. “Judges, juries and adversaries can sense a lack of conviction and are unforgiving with respect to overstatement or misrepresentations. Emphasize the strengths of your case but acknowledge and concede the weak facts or legal precedent. Failing to cite adverse authority or hiding bad facts can be devastating.”

In conclusion

To be sure, the most interesting parts of Mueller’s investigation are likely happening far from public view. Most of the coverage of the probe has focused on its criminal component. But Mueller’s top priority is likely a counterespionage operation, which James Comey confirmed was underway when he testified before Congress (and before his firing).

Naveed Jamali, a former double agent for the FBI who dealt with Russian espionage in the U.S., said this part of the effort won’t necessarily have to do with criminal charges or court proceedings.

“The goal with a counterintelligence operation is to detect and neutralize threats,” said Jamali, author of How to Catch a Russian Spy. “That’s it. If you apply that to the Mueller probe, anything that was used by the Russians against us during the election is a threat that has to be neutralized. That doesn’t mean that it has to be brought to court.”

Simply proving, beyond a shadow of a doubt, who interfered with the 2016 election on behalf of Russia and how they did it would be a significant success for the probe, he added.

“The legal part of this is so fucking boring,” he added. “This is a counterintelligence operation first and foremost.”