U.S. President Donald Trump acknowledged for the first time on Friday that he was under investigation over the firing of FBI Director James Comey, while also appearing to attack Deputy Attorney General Rod Rosenstein.

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

Trump did not specify who he was referring to, but he appeared to mean Rosenstein, the No. 2 official at the U.S. Justice Department. Rosenstein appointed Robert Mueller on May 17 as special counsel to head the inquiry into the Russia matter.

Rosenstein was also the author of a letter in May to Trump criticizing the performance of FBI Director James Comey. While the Trump administration initially said that letter was the reason the president fired Comey on May 9, Trump later said he did so because of the "Russia thing."

Comey told a Senate panel last week he believed Trump fired him because of the Federal Bureau of Investigation's Russia probe. Comey also testified Trump had directed him to drop a related FBI investigation into former national security adviser Michael Flynn.

The tweet was the first explicit acknowledgement by Trump that Mueller was looking into suspicions that the president's firing of Comey was an attempt to obstruct the Russia inquiry.

The Washington Post, which broke the story of Mueller's investigation of Trump on Wednesday, said the investigation was launched shortly after Trump fired Comey on May 9.

At the same time, ABC News reported that Rosenstein has privately acknowledged he may need to recuse himself from matters relating to the Russia probe, given that he could become a potential witness in the investigation.

ABC, citing unnamed sources, said Rosenstein told Associate Attorney General Rachel Brand she would have authority over the probe if he were to step aside.

An expanding investigaton

Trump's tweet came as the investigation into his campaign links to Russia and any possible Russian interference in the election is expanding and seems to be moving into new areas outside of the original suspicions.

Sources say the investigation may be expanding into other aspects of Trump’s business affairs, and not just the Russian connections – and this could be what is really worrying him. In addition, the special prosecutor is most likely investigating a number of people extremely close to the president, including his son-in-law and now White House adviser Jared Kushner.

On Thursday, the New York Times reported that Mueller ordered the Trump White House transition team to keep all documents and communications, physical and electronic, related to the Russian electoral investigation – as well as any material related to Ukraine too.