(CNN) President Donald Trump complained in a furious Friday morning tweet that he was being investigated for firing former FBI Director James Comey and seemed to accuse the deputy attorney general of overseeing a "witch hunt" against him.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director!" Trump said on Twitter, calling the probe into him a "Witch Hunt."

The President's tweet sparked immediate speculation about the future of the official, Rod Rosenstein, who wrote a memo recommending Comey be ousted and also later approved the appointment of a special counsel -- Robert Mueller -- to lead the Russia probe.

A person familiar with Trump's tweet said the President was referencing news reports that he is under investigation and was not indicating that he's been personally informed by the special counsel that he is a target of the probe.

The tweet represented a remarkable show of frustration by a serving President, who is making no secret of his emotions as an investigation that started out as an effort to understand Russian meddling in the election appears to be moving ever closer towards his inner circle and narrowing in on his own conduct.

Aides said that the President had decided to take matters into his own hands and believed he was his own best defender in the escalating crisis surrounding the White House.

A person who has spoken to Trump within the last 24 hours told CNN "the man" in the tweet is Rosenstein, who wrote a memo recommending Comey's firing and approved the appointment of a special counsel to look into Russian election meddling.

Trump has been growing increasingly agitated with Rosenstein over the last day or two in private White House meetings and in conversations with friends, multiple people have told CNN.

Trump ignored shouted questions from reporters about the ongoing Russia controversy as he departed the White House Friday for Miami. The questions included whether he had confidence in Rosenstein, whether he plans to fire special counsel Robert Mueller and how he knows he's under investigation by the FBI.

Sarah Huckabee Sanders, the deputy White House press secretary, referred questions about Trump's tweet to the President's private attorney, Marc Kasowitz.

Firing Muller and Rosenstein?

Trump's decision to go after Rosenstein sparked widespread speculation in Washington that he may dismiss the deputy attorney general in yet another attempt to get past the Russia investigation, although the likely effect of such a step would be to ignite a new political firestorm and claims that the President is abusing his power and mounting a coverup.

"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," warned Sen. Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, in a statement.

"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," she added. "That's undemocratic on its face and a blatant violation of the President's oath of office."

Probe moving closer to Trump

Trump's tweet Friday reflected the first official confirmation that he is being investigated by the special counsel for the reasons he fired Comey last month. Trump conceded in an interview after the episode that he was thinking about the Russia controversy when he made that decision.

Earlier this week, the investigation led by Mueller appeared to move closer to Trump. The Washington Post reported this week that Mueller is investigating whether Trump attempted to obstruct justice when he suggested Comey drop an FBI investigation into Michael Flynn, Trump's former national security adviser, as well as when he fired Comey.

Investigators in Mueller's office are set to interview Director of National Intelligence Dan Coats and National Security Agency Director Adm. Mike Rogers, a source familiar with the matter has told CNN. Reports have alleged that Trump asked both men to publicly quash the credibility of the Russia investigation.

Trump demonstrated Friday morning that the Russia probe continues to consume his attention, tweeting a stream of messages decrying the controversy.

"After 7 months of investigations & committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!" he wrote. "Despite the phony Witch Hunt going on in America, the economic & jobs numbers are great. Regulations way down, jobs and enthusiasm way up!"

'A political fight'

White House aides have been instructed to not talk about Russia investigation, but that doesn't apply to the President.

He has decided -- after consulting with a small circle of advisers in multiple meetings over the last few days -- that "this is a political fight and he's going to fight it," the official said.

He's been advised of the legal ramifications from making these presidential statements on Twitter, but he and at least a few advisers have come to believe the political fight is more urgent. The legal one will come later.

"He's driving this," the White House official said, speaking on condition of anonymity to discuss West Wing internal deliberations.

Friday's tweets have taken other aides across the White House by surprise. But it's part of the President's mindset that he's under siege and he's going to fight like he always has.