WASHINGTON – In his first public comments on the Russia investigation, special counsel Robert Mueller pointedly refused to clear President Donald Trump of criminal wrongdoing, but said charging him with obstruction was "not an option" because of Justice Department policy against prosecuting a sitting president.

In a 10-minute statement delivered from the Justice Department on Wednesday morning, Mueller defended the investigation he supervised, repeated its central conclusions, said it was unnecessary that he testify before Congress and announced that he was leaving the department and closing his office. His remarks largely echoed the text of the 448-page report he submitted in March, but this time he delivered them himself, on camera and in public.

For the first time, in his own voice, Mueller recounted his findings, saying Russia launched a "concerted" effort to interfere with the election. "There were multiple, systematic efforts to interfere in our election, and that allegation deserves the attention of every American," he said.

Mueller said the inquiry into Russia's efforts was one of "paramount importance," so investigators took seriously efforts by Trump and others to thwart their work. But he said the department's Office of Legal Counsel has prohibited the prosecution of a sitting president and his team of prosecutors was bound to follow that rule.

Still, Mueller said if prosecutors had confidence that the president clearly didn't commit a crime, "we would have said that."

Trump tweeted after the statement that nothing had changed from Mueller's report.

"There was insufficient evidence and therefore, in our Country, a person is innocent," Trump said. "The case is closed!"

Mueller's final report described wide-ranging efforts by the Russian government to intercede in the 2016 election on Trump's behalf, but said investigators did not find sufficient evidence to establish a conspiracy with Trump's campaign. Mueller made no decision on whether to press charges of obstruction of justice, despite detailing 10 episodes in which investigators said the president tried to thwart their work.

In his report and again Wednesday, Mueller framed that decision as being less about evidence than about the government's rules for accusing the president of a crime.

Justice Department policy "clearly permits the investigation of a sitting president because it is important to preserve evidence while memories are fresh and documents are available," Mueller said. "Among other things, the evidence could be used if there were co-conspirators who could now be charged."

And he said Congress has its own tools for examining the president's conduct. "The Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing," Mueller said. That process is impeachment.

Then Mueller said he would have nothing more to say on the subject, that the special counsel's office was closing and that he was resigning from the Justice Department. If called to testify before Congress, he said, he would repeat only what was in his office's final report.

White House press secretary Sarah Sanders said Mueller "is moving on with his life, and everyone else should do the same."

Mueller's statement came at a time when more House Democrats are calling for Trump's impeachment, though Speaker Nancy Pelosi, D-Calif., has not approved taking that step. Rep. David Cicilline, D-R.I., who sits on the Judiciary Committee, said Wednesday the next step is to open an impeachment inquiry, to allow the collection of evidence and compel the attendance of witnesses.

Attorney General William Barr, who consulted with other lawyers at the department, decided at the end of Mueller's investigation that no obstruction charges were warranted, even if they could be brought. Barr was confirmed near the end of the inquiry that had been overseen by Deputy Attorney General Rod Rosenstein, who appointed Mueller. Barr has been criticized for his initial four-page summary of the report's conclusions and for statements he made in releasing a redacted version of the report.

But Barr defended his handling of the report before the Senate Judiciary Committee. Trump has said the report completely exonerated him and that the investigation was a partisan witch hunt and attempted coup.

House Democrats were eager to hear from Mueller. The chairman of the House Judiciary Committee, Rep. Jerry Nadler, D-N.Y., has been negotiating for Mueller’s testimony at a public hearing, but said Mueller preferred to appear behind closed doors to avoid a public spectacle. The panel already found Barr in contempt for refusing to provide Congress an unredacted version of the report.

Nadler said it is up to Congress to respond to "crimes, lies and other wrongdoing by President Trump."

"Although Department of Justice policy prevented the special counsel from bringing criminal charges against the president, the special counsel has clearly demonstrated that President Trump is lying about the special counsel’s findings, lying about the testimony of key witnesses in the special counsel’s report, and is lying in saying that the special counsel found no obstruction and no collusion," Nadler said Wednesday.

But Senate Majority Leader Mitch McConnell, R-Ky., has declared the Russia probe over and said "case closed." The chairman of the Senate Judiciary Committee, Sen. Lindsey Graham, R-S.C., has said he would ask Mueller whether he disagreed with the way Barr characterized the report, but that Mueller needn’t necessarily testify. A Republican member of the panel, Sen. John Kennedy of Louisiana, said if Mueller chooses to testify, he should do so in public.

Mueller said nobody in the administration told him whether to testify. But he said he couldn't say anything more publicly than what is included in the report.

"The work speaks for itself," Mueller said. "The report is my testimony."

More:Sen. Lindsey Graham reacts to Mueller statement on Russia probe: 'The case is closed'