https://www.newsweek.com/robert-mueller-special-counsel-russia-comey-fbi-611296

Russia Investigation: Robert Mueller 'Fiercely Independent' and Aggressive on Getting Results, FBI Insiders Say

Robert Mueller's first test as FBI director came just one week after he took the job, on September 11, 2001. The attacks thrust the Marine Corps veteran, who had been a Department of Justice official and a United States attorney, into the new position and transformed his bureau's mission. The day after the attacks, Mueller told the heads of all 56 FBI field offices that the bureau would be embarking on the biggest investigation in its history. "FBI officials said they knew this probe was different from anything else they'd ever done," Newsweek reported at the time.

The event would shape Mueller's tenure as the nation's top law enforcement officer, a position he held longer than any other FBI director besides J. Edgar Hoover. President George W. Bush appointed him to the position in 2001 and he left in 2013, after President Barack Obama asked him to continue leading the bureau for two years beyond the 10-year term limit.

Now he will again serve under the Justice Department, as special counsel overseeing the FBI's investigation into Russia's meddling in the 2016 president election. Lawmakers had called for special counsel in the wake of President Donald Trump's firing of Mueller's successor at the FBI, James Comey.

Related: Can Comey untangle the Trump-Russia allegations?

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In interviews with Newsweek, former FBI officials who served under both Mueller and Comey describe their leadership styles as night and day. Both men are generally respected within the broad FBI community, but for different reasons, according to the former officials.

David Johnson, who retired in December as the associate executive assistant director for the FBI's Criminal, Cyber, Response, and Services Branch, told Newsweek in March that Mueller likely honed his leadership skills as a Marine in Vietnam. "He is incredibly effective," Johnson said.

Mueller is also known for having shaken up the bureau following the September 11 attacks in order to respond to new kinds of crimes and threats, particularly terrorism. Those efforts did not sit well with every agent. "Some of that was well-accepted and a lot of it was a challenge because people don't like change," Frank Montoya, Jr., a former special agent in charge of the FBI's Seattle division and the former head of national counterintelligence for the bureau, told Newsweek in March. "[But] it was absolutely necessary."