President Donald Trump acknowledged on Friday that he is under investigation for firing FBI Director James Comey, and appeared to attack his own deputy attorney general for launching a special counsel probe that has intensified in recent days.

“I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt,” Trump tweeted.

The president did not immediately clarify the tweet, which likely refers to Deputy Attorney General Rod Rosenstein, but could also refer to special counsel Robert Mueller, who is overseeing the larger investigation into Russian meddling in the 2016 presidential election and possible collusion between Trump’s campaign and the Kremlin.

Rosenstein is overseeing the probe by Mueller, which reportedly has expanded to include whether Trump obstructed justice when he fired Comey and allegedly pressured intelligence officials to downplay or undermine the Russia investigation.

Trump’s unusual tweet prompted a sharp rebuke from the top Democrat on the Senate Judiciary Committee, which oversees matters related to the Justice Department.

“I’m growing increasingly concerned that the president will attempt to fire not only Robert Mueller, the special counsel investigating possible obstruction of justice, but also Deputy Attorney General Rosenstein who appointed Mueller,” Sen. Dianne Feinstein (D-Calif.) said in a statement.

Feinstein said Trump’s tweets suggest he lacks respect for the rule of law and that they amount to a “a blatant violation of the president’s oath of office.” “[I]f the president thinks he can fire Deputy Attorney General Rosenstein and replace him with someone who will shut down the investigation, he’s in for a rude awakening. Even his staunchest supporters will balk at such a blatant effort to subvert the law,” she said.

Trump’s tweet appeared to be a reference to a memo Rosenstein issued shortly after assuming his position in the DOJ. In the memo, Rosenstein issued a blistering assessment of Comey’s performance as FBI director, pointing to his unusual handling of last year’s Hillary Clinton email investigation. But Rosenstein’s memo didn’t explicitly recommend Comey’s firing, and Rosenstein himself later said he wrote the memo after learning Trump already intended to fire Comey.

Trump himself said he had already planned to fire the FBI director, citing in part the pressure of the Russia investigation on his White House.

Rosenstein’s memo, though, did strongly suggest that a change in FBI leadership would be beneficial.

"Although the President has the power to remove an FBI director, the decision should not be taken lightly. I agree with the nearly unanimous opinions of former Department officials," he wrote. "The way the Director handled the conclusion of the email investigation was wrong. As a result, the FBI is unlikely to regain public and congressional trust until it has a Director who understands the gravity of the mistakes and pledges never to repeat them. Having refused to admit his errors, the Director cannot be expected to implement the necessary corrective actions.”

Though Rosenstein himself authorized Mueller’s investigation, he testified last week that Mueller has complete independence to pursue the matter as he sees fit. Some of Trump’s allies have encouraged the president to fire Mueller, but Rosenstein has insisted he is the only one with authority to do so, and that he believes Mueller — widely respected by both parties in Washington — is doing his job properly.

Immediately after Comey’s ouster, Feinstein called for Rosenstein, too, to recuse himself from the Russia investigation and for a career DOJ official to handle the matter instead. But Rosenstein’s appointment of Mueller appeared to calm nerves on both sides of the aisle. Democrats indicated they were comfortable with Rosenstein overseeing the probe, if only because they trusted Mueller to report any potential interference.

On Friday, though, the Democratic National Committee called for Rosenstein’s recusal and urged DOJ to bypass other Trump appointees and hand the matter to a career official.

Reports emerged earlier this week suggesting Mueller’s probe had begun looking at the prospect of obstruction by Trump himself. Trump fired Comey on May 9 while Comey was overseeing the initial Russia investigation. Comey has since testified that Trump had asked him repeatedly to publicly announce he was not under investigation. Comey gave him private assurances that he was not a target of the investigation at that stage. Now, though, Trump himself acknowledges he’s being investigated by the special counsel.

And as a result, Trump appears to have turned his ire toward Rosenstein, who he nominated to the second-highest post in the Justice Department earlier this year. Though typically, the attorney general would have the authority to hire or dismiss a special counsel, Attorney General Jeff Sessions recused himself in March from any decision-making authority over the Russia probe.

Trump’s Twitter attack comes a day after Rosenstein issued a cryptic statement, urging Americans not to believe news stories that cite undisclosed sources.

“Americans should exercise caution before accepting as true any stories attributed to anonymous ‘officials,’ particularly when they do not identify the country — let alone the branch or agency of government — with which the alleged sources supposedly are affiliated. Americans should be skeptical about anonymous allegations. The Department of Justice has a long-established policy to neither confirm nor deny such allegations,” the statement read.