WASHINGTON — The Justice Department on Wednesday appointed former FBI Director Robert Mueller as a special counsel to oversee a federal investigation into potential coordination between Russia and the Trump campaign during the 2016 presidential election.

The appointment came amid a growing Democratic outcry for someone outside the Justice Department to handle the politically charged investigation.

It followed the revelation Tuesday that fired FBI Director James Comey had written in a memo that Trump, in a February meeting in the Oval Office, had asked him to end an investigation into former national security adviser Michael Flynn. The White House has denied that account.

Mueller was appointed FBI director in 2001 and led the FBI through the aftermath of the Sept. 11 terror attacks. He retired in 2013.

The Justice Department said Mueller has resigned from his job at a private law firm to take the job of special counsel.

The Justice Department declined to explain the decision-making involved in the appointment.