The Justice Department on Wednesday named a special prosecutor to investigate Russian interference in the 2016 presidential election and possible collusion between the Trump campaign and Russian operatives, CNN reports.

Deputy Attorney General Rod Rosenstein announced Wednesday that former FBI Director Robert Mueller will head the Justice Department’s investigation. Rosenstein took over the department’s Russia probe after Attorney General Jeff Sessions recused himself following intense scrutiny over his ties to the Trump campaign.

In a letter obtained by CNN, the Justice Department authorized Mueller “to prosecute federal crimes arising from the investigation of these matters.”

“In my capacity as acting attorney general, I determined that it is in the public interest for me to exercise my authority and appoint a Special Counsel to assume responsibility for this matter,” Rosenstein said in a statement.

“What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command,” he added.

CNN’s Carol Costello reports that the White House received less than one hour notice that a special counsel had been appointed.

#URGENT: White House received less than one hour notice on Special Counsel

— Carol Costello (@CarolCNN) May 17, 2017

According to NBC News, former Trump aides Michael Flynn and Paul Manafort are key figures in the investigation. The FBI is examining contacts, money transfers and business relationships between Trump associates and Russian operatives.

Last week, Rosenstein said there was no need for a special prosecutor in the Russia probe. In the days since, reports emerged that Trump asked fired FBI director James Comey to stop investigating Flynn, his former national security adviser. Those reports intensified calls for a special counsel in the investigation.