In a message that journalists and political observers say looks like a shocking attack by a sitting president on his own Justice Department, President Donald Trump appeared to take aim at Deputy Attorney General Rod Rosenstein in a Friday morning tweet. Trump also acknowledged, for the first time publicly, that he is under federal investigation.

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

When Trump fired former FBI Director James Comey last month, he initially cited a memo written by Rosenstein as the basis for the dismissal. However, Trump later told NBC News' Lester Holt that he had already decided to fire Comey before that memo was written.

Hans Nichols of MSNBC, reporting from the White House, said the tweet is remarkable for two reasons. First, it is the president himself acknowledging that he is, in fact, under investigation for possible obstruction of justice—something even on Thursday the White House would not acknowledge. And second, said Nichols, the tweet appears to be Trump "attacking the Justice Department."

In a new tweet, Pres. Trump appears to confirm he's under investigation for obstruction of justice. https://t.co/paqrfU0Rmo

— MSNBC (@MSNBC) June 16, 2017

Trump's tweet Friday morning comes in the wake of news this week that Special Counselor Robert Mueller III has expanded his probe into Russian meddling in last year's election to include a look at whether or not the president attempted to obstruct justice by interfering with federal agencies looking into the matter.

David Gergen, a political analyst for CNN, said that Trump's legal team must be "going a little nuts" at this point. "They clearly don't have control over what he's tweeting and it has been long standing practice that when you're under investigation you don't attack publicly the investigators. It only stirs them up."

The most important takeaway from the president's tweet, Gergen continued, "is that the president has launched a campaign to discredit the special counsel and to discredit the investigations. It could be a prelude to firing Rosenstein. It could be a prelude to firing the special counsel."

Amid stirrings earlier this week that Trump was possibly considering the idea of firing Mueller, it was Rosenstein who testified before a congressional hearing that such an act could not be taken without specific cause. And as Rep. Ted Lieu (D-Calif.) declared in a tweet Thursday, "If Trump fired Mueller (he'd also have to fire Dep AG Rosenstein), it would be an 11 on the Richter scale of #obstructionofjustice."

President confirms he is the subject of an inquiry into obstruction of justice and abuses of power. A pair of VERY impeachable offenses. https://t.co/nxPAmqoMir

— John Nichols (@NicholsUprising) June 16, 2017

If Trump fired Mueller (he'd also have to fire Dep AG Rosenstein), it would be an 11 on the Richter scale of #obstructionofjustice. https://t.co/ElDcUzmMNK

— Ted Lieu (@tedlieu) June 15, 2017