The Justice Department appointed former FBI Director Robert Mueller as special counsel to head the federal investigation into Russia’s interference in the 2016 presidential election, the office announced Wednesday evening. The investigation will also look into possible collusion between the Trump campaign and Russian officials.

The appointment comes a week after President Trump fired FBI Director James Comey and a day after the New York Times reported that Trump asked Comey to drop an inquiry into former national security advisor Michael Flynn, who resigned after lying about communications with the Russian ambassador to the U.S.

Deputy Attorney General Rod Rosenstein — who wrote one of the recommendation letters Trump originally cited as justification for firing Comey — appointed Mueller, VICE News can confirm. Attorney General Jeff Sessions, who would normally have the duty of appointing a special counsel, recused himself from the Russia investigation in March following the revelation that he failed to disclose conversations with a Russia ambassador to Congress.

“I determined that it is in the public interest for me to exercise my authorities and appoint a special counsel to assume responsibility for this matter,” Rosenstein said in a statement. “My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination.”

Mueller was Comey’s immediate predecessor at the FBI, which he led from 2001 to 2013 under both George W. Bush and Barack Obama. Though FBI directors typically serve for 10 years in order to avoid partisan politics, Obama asked Mueller to stay on an extra two years in 2011. Republican Sen. James Lankford, of Oklahoma, told Fox News that appointing Mueller would help “expedite” the FBI’s investigation, because Mueller knows the bureau.

Mueller also resigned Wednesday from his position at the law firm WilmerHale to take on the special counsel role.

Following Comey’s firing, Democrats have clamored for an investigation that would be seen as less partisan. There are two main methods to accomplish this. Following the attacks on September 11, 2001, Congress convened outside experts to form an independent commission. And after the 2012 Benghazi attack that led to the death of a U.S. ambassador, members of Congress joined a select committee. But both independent commissions and select committees can only generate reports about their findings. An investigation headed by a special counsel, on the other hand, can result in criminal charges.

In the past, the Department of Justice has appointed outside individuals to lead investigations like Watergate, the “Whitewater” investigation into President Bill Clinton and Hillary Clinton’s real estate dealings, and the 2003 investigation into the leak of the identity of a former CIA operative. The last special counsel was appointed in September 1999, to investigate potential misconduct by federal law enforcement during the 1993 siege of the Branch Davidian compound near Waco, Texas.

Rosenstein is also set to brief all 100 senators about Comey’s firing on Thursday.