Who is special counsel and former FBI director Robert Mueller?

Robert Mueller, who was named in late May as the special counsel to spearhead the FBI's investigation into Russian connections to the 2016 election "and related matters," has been in the headlines more than once since he was appointed.

Most recently, President Donald Trump said if Mueller investigated his and his family's personal finances, specifically those unrelated to Russia, it would cross a line.

For now, however, Mueller's role is safe. Principal deputy press secretary Sarah Huckabee Sanders, who was named press secretary on July 21 after Sean Spicer resigned, later told reporters that Trump "has no intention" of firing Mueller.

Mueller has a long history with the FBI, beginning in 2001, when he was nominated FBI director by President George W. Bush and assumed the position just seven days before the 9/11 terrorist attacks.

Mueller, 72, went on to serve for 12 years, from September 2001 until September 2013, carrying the title for second-longest serving FBI director. He was succeeded by James Comey, who was fired abruptly by Trump on May 9.

Mueller and Comey worked together in the past: The two were together during the dramatic hospital showdown in 2004 when they wanted to stop then-Attorney General John Ashcroft from reauthorizing then-President George Bush's domestic surveillance program. Comey testified about the incident in front of the Senate Judiciary Committee in 2007, and it is widely cited as an example of Comey's political independence.

When he left the bureau and his position was assumed by Comey, who was appointed by then-President Barack Obama, Mueller shared kind words about his successor.

"I have had the opportunity to work with Jim for a number of years in the Department of Justice, and I have found him to be a man of honesty, dedication and integrity. His experience, his judgment and his strong sense of duty will benefit not only the bureau, but the country as a whole," Mueller said at the announcement ceremony in the White House's Rose Garden.

Mueller was later criticized by Trump, who said in a "Fox and Friends" interview on June 23 that it was "bothersome" how close Comey and Mueller were. The hosts of the show had asked if their relationship warranted Mueller's recusal. "Well, he is very, very good friends with Comey, which is very bothersome," Trump responded. "We're going to have to see."

In the same interview, the president claimed that Mueller's team of lawyers are "all Hillary Clinton supporters." Mueller, who served in the independent role of FBI director under both Republican and Democratic presidents, has been a registered Republican through most of his career.

His appointment as special counsel is not the first time Mueller was given a spotlight for his investigative work.

NFL Commissioner Roger Goodell put Mueller in charge of the investigation into the domestic assault incident involving Baltimore Ravens' running back Ray Rice in 2014.

According to his profile on the FBI website, prior to joining the FBI as its director, Mueller served in the Marine Corps and was a part of the rifle platoon in Vietnam. His profile says that he has received the Bronze Star, two Navy Commendation medals and a Purple Heart.

He went to Princeton University for his undergraduate degree and the University of Virginia for law school after his military service.

Mueller then worked as a litigator in San Francisco before working in the U.S. Attorney's Office in California's Northern District and later in Boston, where he worked as assistant U.S. attorney, according to the FBI profile.

He returned to U.S. attorney's office in San Francisco in 2001 before being nominated for his job as director at the FBI. The Department of Justice reported that Mueller resigned from his law firm in order to avoid any conflicts of interest with firm clients or attorneys.

ABC News' Cheyenne Haslett contributed to this report.
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