President Trump issued a statement Wednesday evening:

As I have stated many times, a thorough investigation will confirm what we already know—there was no collusion between my campaign and any foreign entity. I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country.

The decision to appoint Mueller, whose formal title is special counsel, was signed by Rod Rosenstein, who is deputy attorney general but made the appointment as acting attorney general. That’s because Attorney General Jeff Sessions has been forced to recuse himself from the Russia probe, after admitting he failed to disclose meetings with Russian Ambassador Sergey Kislyak to the Senate.

Rosenstein wrote in an order that Mueller is authorized to take over the investigation that Comey confirmed to Congress in a March hearing. That includes “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump” and “any matters that arose or may arise from directly from the investigation.” It also gives Mueller authority to look into other crimes noted under a statute that establishes the special counsel, “such as perjury, obstruction of justice, destruction of evidence, and intimidation of witnesses; and to conduct appeals arising out of the matter being investigated and/or prosecuted.”

If he believes it is necessary, Mueller “is authorized to prosecute federal crimes arising from the investigation of these matters.”

Mueller is the second-longest serving director in FBI history. He was appointed by President George W. Bush and then given an unusual additional short term by President Barack Obama. The appointment quickly won praise from observers of the administration.

“It's an excellent choice,” said Benjamin Wittes, a senior fellow at the Brookings Institution and editor of Lawfare. “It's good that he did it. And it's an important first step in salvaging a terrible situation.”

Wittes last week wrote a fiery denunciation of Rosenstein for his role in the Comey firing, and had questioned the deputy attorney general’s honor and independence.

Amy Jeffress, a former counsel to Attorney General Eric Holder, called Mueller “the best choice,” citing his “rock solid integrity” and his smart, no-nonsense approach.

John Q. Barrett, a professor of law at St. John's University and a former associate counsel in the office of the Iran-Contra independent counsel, described Mueller as a "perfect fit."

“He's managed and supervised big, complex investigations, including in the national security area,” said Barrett. “He’s had from the line level, to the highest management level, investigative and prosecutorial roles in his background that are suited for this. He’s attorney-general material, and this job is really a stand in for the attorney general.”