Updated

The Justice Department's No. 2 official was on Capitol Hill Thursday to brief senators about his decision to appoint former FBI Director Robert Mueller to oversee the investigation into Russian meddling in the 2016 election.

Deputy Attorney General Rod Rosenstein is holding a rare closed-door session with the full Senate. He is expected to discuss not only his appointment of an independent special counsel for the Russia investigation but also his role in the firing of FBI Director James Comey by President Trump as well as allegations that Trump asked Comey to end a criminal investigation into ex- National Security Adviser Michael Flynn.

Sen. Chris Coons, D- Del., told The Washington Post that Rosenstein’s recommendation to Trump to fire Comey puts his “credibility is on a very shaky foundation with many of my colleagues. He needs to be forthright, thorough, responsive to have any hope of restoring the confidence of many of my Democratic colleagues.”

The Senate briefing was on the books before Rosenstein announced Wednesday that he had named Mueller to take over the investigation.

In a statement, Rosenstein said he decided to hand off the high-profile investigation to an independent investigator, “in order for the American people to have full confidence in the outcome.”

“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. "My decision is not a finding that crimes have been committed or that any prosecution is warranted.”

The appointment gives Mueller, who led the FBI through the Sept. 11 terrorist attacks and served under presidential administrations of both parties, sweeping powers to investigate whether Trump campaign associates colluded with the Kremlin to influence the outcome in his behalf, as well as the authority to prosecute any crimes uncovered during the probe.

"I accept this responsibility and will discharge it to the best of my ability," Mueller said in a statement.

The broad mandate, beyond any specific Trump-Russia connection, also covers "any matters that arose or may arise directly from the investigation."

Senior DOJ officials told Fox News the White House was informed of the decision for a special counsel after the order had been signed, through the White House Counsel’s office, about an hour before the decision went public.

"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," President Trump said in a statement in reaction to the DOJ announcement. "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 Justice Department said Mueller has resigned from his job at a private law firm to take the job of special counsel.

During a press briefing Wednesday morning, House Speaker Paul Ryan shrugged off questions from reporters about whether the country would be better off with Vice President Mike Pence in the Oval Office.

"Oh good grief! I'm not even going to give credence to that," he said.

The Associated Press contributed to this report.