• Trump voluntarily releases a financial disclosure,

may now be part of the probe. Thursday night, Mr. Rosenstein issued an unusual statement complaining about leaks in the case.

Trump advisers and confidants describe the president as increasingly angry over the investigation, yelling at television sets in the White House carrying coverage and insisting he is the target of a conspiracy to discredit — and potentially end — his presidency. Some of his ire is aimed at Mr. Rosenstein and investigative special counsel Robert Mueller, both of whom the president believes are biased against him, associates say.

Dianne Feinstein, top Democrat on the Senate Judiciary Committee, said she was “increasingly concerned” that Mr. Trump will fire both Mr. Mueller and Mr. Rosenstein.

“The message the president is sending through his tweets is that he believes the rule of law doesn’t apply to him, and that anyone who thinks otherwise will be fired,” Ms. Feinstein said. “That’s undemocratic on its face and a blatant violation of the president’s oath of office.”

Aides have counseled the president to stay off Twitter and focus on other aspects of his job. They have tried to highlight the positive reviews he received Wednesday when he made a statesman-like appearance in the White House to address the nation after Rep. Steve Scalise was shot during a congressional baseball practice.

Yet Mr. Trump’s angry tweets Friday underscored the near-impossible challenge his advisers and legal team have been trying to get him to avoid weighing in on anactive probe.

The president has denied that he has any nefarious ties to Russia and has also disputed that he’s attempted to block the investigation into his campaign’s possible role in Russia’s election-related hacking. It was unclear whether his tweet about being under investigation was based on direct knowledge or new media reports that suggest Mr. Mueller is examining whether the president obstructed justice by firing Mr. Comey.

The tweets came shortly after Mr. Rosenstein issued his unusual statement that appeared to be warning about the accuracy of such reports.

“Americans should be skeptical about anonymous allegations,” Mr. Rosenstein said. “The Department of Justice has a long-established policy to neither confirm nor deny such allegations.”

The department would not comment on the record on whether Mr. Trump, who has repeatedly complained about leaks on the case, requested the statement. But a department official speaking anonymously said Mr. Rosenstein acted on his own.

Mr. Trump has told associates he has the legal authority to fire Mr. Mueller. What is clear is that he could order the Justice Department to ax Mr. Mueller, which may result in Mr. Rosenstein’s departure and would certainly intensify the uproar over the investigation. Many in Mr. Trump’s orbit — including his son Donald Trump Jr. and adviser Newt Gingrich — have deemed Mr. Mueller biased and worthy of dismissal.

Mr. Rosenstein has been overseeing the Russia probe since shortly after Attorney General Jeff Sessions recused himself. But Mr. Rosenstein, too, may ultimately have to hand off oversight given his role in Mr. Trump’s decisionto fire Mr. Comey.

Mr. Trump’s tweets came after the top lawyer for his transition team warned the organization’s officials to preserve all records and other materials related to the Russia probe. An official of Mr. Trump’s transition confirmed the lawyer’s internal order, which was sent Thursday.