WASHINGTON (AP) — President Donald Trump blasted special counsel Robert Mueller on Thursday, calling him a "never Trumper" who led a biased investigation on Russia's interference in the 2016 election and failed to investigate his opponents who didn't want Trump to be president.

Trump's eruption came a day after Mueller pointedly rejected his repeated claims that he was cleared of obstruction of justice allegations and that the two-year inquiry was merely a "witch hunt."

The president also offered mixed messages on Russia's efforts to help him defeat Democrat Hillary Clinton, first tweeting that he had "nothing to do with Russia helping me get elected," then minutes later, telling reporters: "Russia did not help me get elected."

Trump said Mueller, who is a Republican, was "conflicted" and should have investigated law enforcement officials who the president claims tried to undermine him.

"Robert Mueller should have never been chosen," Trump said, adding falsely that Mueller wanted the FBI director job, but the president told him no. "I think Mueller is a true never Trumper. He's somebody who didn't get a job that he wanted very badly."

Mueller, who was appointed special counsel by Trump's Justice Department, was previously FBI director, appointed by Republican President George W. Bush.

Speaking to reporters on the White House South Lawn, Trump insisted that he's been tough on Russia and that Moscow would have preferred Hillary Clinton as president. The special counsel's report said Russian interference in the election helped Trump defeat Clinton,

Asked about impeachment by Congress, he called it a "dirty word" and said he couldn't imagine the courts allowing him to be impeached. "I don't think so because there's no crime," he said.

Mueller said Wednesday that charging Trump with any crime in court was "not an option" because of federal rules, but he used his first public remarks on the Russia investigation to emphasize that he did not exonerate the president.

"If we had had confidence that the president clearly did not commit a crime, we would have said so," Mueller declared.

The special counsel's remarks on indicting Trump marked a counter to criticism, including by Attorney General William Barr, that Mueller should have reached a determination on whether the president illegally tried to obstruct the probe by taking actions such as firing FBI Director James Comey.

Mueller made clear that his team never considered indicting Trump because the Justice Department prohibits the prosecution of a sitting president.

"Charging the president with a crime was therefore not an option we could consider," Mueller said during a televised statement .

He said he believed such an action would be unconstitutional.

Mueller did not use the word "impeachment" but said it was the job of Congress, not the criminal justice system, to hold the president accountable for any wrongdoing.

The special counsel's statement largely echoed the central points of his lengthy report, which was released last month with some redactions. But his remarks, just under 10 minutes long and delivered from a Justice Department podium, were extraordinary given that he had never before discussed or characterized his findings and had stayed mute during two years of feverish public speculation.

Mueller said his work was complete and he was resigning to return to private life. Under pressure to testify before Congress, Mueller did not rule it out. But he seemed to warn lawmakers that they would not be pulling more detail out of him. His report is his testimony, he said.

"So beyond what I have said here today and what is contained in our written work," Mueller said, "I do not believe it is appropriate for me to speak further about the investigation or to comment on the actions of the Justice Department or Congress."

His remarks underscored the unsettled resolution, and revelations of behind-the-scenes discontent, that accompanied the end of his investigation. His refusal to reach a conclusion on criminal obstruction opened the door for Barr to clear the Republican president, who in turn has cited the attorney general's finding as proof of his innocence. Mueller has privately vented to Barr about the attorney general's handling of the report, while Barr has publicly said he was taken aback by the special counsel's decision to neither exonerate nor incriminate the president.

New York Rep. Jerrold Nadler said it falls to Congress to respond to the "crimes, lies and other wrongdoing of President Trump — and we will do so." House Speaker Nancy Pelosi has so far discouraged members of her caucus from demanding impeachment, believing it would only help Trump win re-election and arguing that Democrats need to follow a methodical, step by step approach to investigating the president. But she hasn't ruled it out.

Trump has blocked House committees' subpoenas and other efforts to dig into the Trump-Russia issue, insisting Mueller's report has settled everything.

The report found no criminal conspiracy between the Trump campaign and Russia to tip the outcome of the 2016 presidential election in Trump's favor. But it also did not reach a conclusion on whether the president had obstructed justice.

Barr has said he was surprised Mueller did not reach a conclusion, though Mueller in his report and again in his statement Wednesday said he had no choice. Barr and Deputy Attorney General Rod Rosenstein then stepped into the void, deciding on their own that the evidence was not sufficient to support a criminal charge.

"Under longstanding department policy, a president cannot be charged with a federal crime while he is in office," Mueller said. "That is unconstitutional. Even if the charge is kept under seal and hidden from public view that, too, is prohibited."

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Associated Press writers Mary Clare Jalonick, Lisa Mascaro and Jonathan Lemire contributed to this report.