WASHINGTON >> President Donald Trump lashed out today at the appointment of a special counsel to investigate allegations that his campaign collaborated with Russia to sway the 2016 election, tweeting that it is “the single greatest witch hunt of a politician in American history!”

Trump has made similar complaints before, but this one came the day after the Justice Department appointed former FBI Director Robert Mueller to lead the federal Trump-Russia investigation. Mueller will have sweeping powers and the authority to prosecute any crimes he uncovers. But Trump’s claim ignored impeachment efforts and blistering verbal attacks on previous presidents and other political leaders.

Meanwhile, Sen. Richard Burr of North Carolina, who chairs the intelligence committee, created confusion on Capitol Hill by saying that a lawyer for Michael Flynn had told the panel Flynn would not be complying with the panel’s subpoena for documents. Flynn, Trump’s fired national security adviser, is a key figure in the Trump-Russia investigation.

Shortly thereafter Burr reversed himself in a statement saying Flynn’s attorneys “have not yet indicated their intentions regarding the Senate Intelligence Committee’s subpoena,” and he would welcome “their willingness to cooperate.” It was not clear what caused the mix-up.

Flynn, through his lawyer, had earlier asked for immunity from “unfair prosecution” in exchange for agreeing to cooperate with the committee. A key adviser to Trump during the presidential campaign, Flynn was dismissed not long after the inauguration when it became known that he had had Russian contacts and had misled the Trump team about them.

On a day of fast-moving developments, the House intelligence committee announced that it, too, had asked for documents, in this case from the FBI and the Justice Department.

The broad federal probe under Mueller, a lawman who engenders deep bipartisan respect, was just getting underway.

His appointment, by Deputy Attorney General Rod Rosenstein, was a striking shift for the Justice Department, which had resisted increasingly insistent calls from Democrats for an outside prosecutor. It immediately escalated the legal stakes — and the potential political damage — for a president who has tried to dismiss the matter as partisan witch hunt and a “hoax.”

The announcement, the latest in the shock-a-day Washington saga, was made a day ahead of Rosenstein’s scheduled closed-door appearance this afternoon before the Senate. Democratic senators had been prepared to press him to appoint a special prosecutor, and his decision defused their complaints, leading to praise instead.

“This was a very good first step. Mr. Rosenstein has done the right thing,” Senate Minority Leader Chuck Schumer of New York said on the Senate floor. “I now have significantly greater confidence that the investigation will follow the facts wherever they lead.”

Trump himself, in a statement right after the appointment late Wednesday, said merely, “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.”

But he expressed more resentment today in his tweets. In addition to complaining he was being abused in unprecedented ways, he took aim anew at his presidential rival, Democrat Hillary Clinton, saying she never faced a special counsel despite “all the illegal acts that took place” in her campaign. He gave no examples.

Mueller’s broad mandate gives him not only oversight of the Russia probe, but also over “any matters that arose or may arise directly from the investigation.” That would surely include Trump’s firing last week of FBI Director James Comey.

Mueller, a former federal prosecutor at the Justice Department, was confirmed as FBI director days before the Sept. 11, 2001, attacks that would ultimately shape his tenure. He was so valued that President Barack Obama asked him to stay on two years longer than his 10-year term.

Comey succeeded him, appointed by Obama.

Rosenstein said the Mueller appointment was “necessary in order for the American people to have full confidence in the outcome.”

Republicans have largely stood behind Trump in the first months of his presidency as the FBI and congressional investigations into Russia’s election meddling intensified. But GOP lawmakers have grown increasingly anxious since Trump fired Comey, who had been leading the bureau’s probe — and after Comey associates said he had notes from a meeting in which Trump asked him to shut down the investigation into Flynn’s Russia ties.

Trump is leaving town Friday on his first foreign trip, and aides have been hopeful the journey will be a chance for the administration to get back on track after weeks of chaos and distractions.

Associated Press writers Deb Riechmann, Eileen Sullivan, Matthew Daly and Mary Clare Jalonick contributed to this report.