President Donald Trump appeared to confirm in a tweet on Friday that he is under investigation for firing former FBI Director James Comey and blamed what he called a "Witch Hunt" on "the man who told me to fire" Comey — a possible reference to the deputy attorney general.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump tweeted. NBC News has reported that federal investigators are examining whether the president attempted to obstruct justice.

I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt — Donald J. Trump (@realDonaldTrump) June 16, 2017

Trump may have been referring to Deputy Attorney General Rod Rosenstein, who wrote a highly critical memo about Comey last month. In separate closed-door meetings with the House and the Senate, Rosenstein said his memo was "not a statement of reasons to justify a for-cause termination."

Trump's tweet comes as NBC News has learned of the following developments: staffers on his 2016 campaign have been asked to preserve documents, text messages and electronic devices; Michael Cohen, the president's lawyer, has hired a private attorney; and Michael Caputo, a former communications adviser on the campaign, has been contacted by the FBI.

The Justice Department declined to comment on Friday on the president's tweet.

A source close to Trump's outside legal counsel said later Friday that the president was not confirming the obstruction probe in his tweet, but was referencing a Washington Post report published Wednesday night that said he was under investigation.

The source said the president has not been notified that he is under investigation.

Trump also added some more legal fire power to his team, hiring John Dowd, a source close to the president’s counsel confirmed. Dowd is experienced handling political cases and investigated Pete Rose for Major League Baseball.

After 7 months of investigations & committee hearings about my "collusion with the Russians," nobody has been able to show any proof. Sad! — Donald J. Trump (@realDonaldTrump) June 16, 2017

Despite the phony Witch Hunt going on in America, the economic & jobs numbers are great. Regulations way down, jobs and enthusiasm way up! — Donald J. Trump (@realDonaldTrump) June 16, 2017

The probe is being led by special counsel Robert Mueller, who was appointed by Rosenstein. Rosenstein has supervised the widening investigation into Russian election interference since Attorney General Jeff Sessions recused himself.

In an interview with NBC's Lester Holt on May 11, Trump said he was "going to fire [Comey] regardless of recommendation" from Rosenstein and Sessions. In a letter to Trump, Sessions called for a "fresh start" at the FBI and recommended Comey's removal.

"What I did is I was going to fire Comey. My decision," Trump told Holt, a day after the White House had said the president fired Comey based on a recommendation from Rosenstein.

As part of his probe into possible obstruction, Mueller has requested interviews with senior intelligence officials about their conversations with Trump, a former senior intelligence official with knowledge of the discussions confirmed to NBC News.

Those who have agreed to be interviewed include: Dan Coats, director of national intelligence; Mike Rogers, chief of the National Security Agency; and Richard Ledgett, who recently left his post as deputy to Rogers.

At a Senate Intelligence Committee hearing last week, Coats and Rogers refused to say whether Trump asked them to intervene in the investigation, which is also examining whether the Trump campaign team colluded with Russia during last year's presidential campaign.

Former senior Trump campaign staff have not been told by Mueller to preserve any records, but they have been advised by Michael Glassner, chief strategist for Trump 2020, who sent a letter to everyone who had worked on the 2016 campaign, to save whatever physical or electronic documents they still have, including text messages. The letter also asks that any campaign electronic devices, like cell phones, be preserved.

Caputo, who had served as a communication adviser to the campaign, has been asked the House Intelligence Committee to testify in mid-July and turn over relevant documents, NBC News has learned. Caputo's lawyer is negotiating to have him to testify in an open hearing.

NBC News also has learned that the FBI has contacted Caputo.