WASHINGTON (AP) — President Donald Trump appeared to confirm Friday that he is under investigation for obstruction of justice, claiming that he is being investigated for firing FBI Director James Comey by the man who told him to do it.

It wasn't clear whether the president was basing his tweet on direct knowledge that he is under investigation, or on reports this week that special counsel Robert Mueller is examining whether the president obstructed justice by firing Comey last month amid the ongoing Russia investigation.

Mueller was appointed as special counsel to investigate Russian involvement in the 2016 presidential election and possible collusion by Trump campaign associates with the Russians, but there has been no indication that Mueller told Trump to fire Comey.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," the president wrote in his tweet.

In another tweet earlier Friday, Trump writes, "After 7 months of investigations & committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!"

Deputy Attorney General Rod Rosenstein submitted a widely circulated memo to the president last month raising concerns over Comey's performance, but Trump later asserted that he had already made the decision himself to fire Comey.

The White House confirmed that Mueller was among the candidates Trump interviewed to replace Comey. A day later, Rosenstein appointed Mueller, who was at the helm of the FBI during the Sept. 11 attacks, as special counsel.

Friday's tweets are the latest in a week of angry social media responses by the president over a report by The Washington Post that Mueller was looking into whether Trump obstructed justice.

In a series of statements on Twitter Thursday, Trump repeated his assertion that Mueller's investigation into Russian interference in the 2016 campaign is a "WITCH HUNT" based on the "phony" premise of possible collusion between Russia and a cadre of Trump campaign associates.

The president complained the probe is unfair and wondered why his defeated Democratic opponent wasn't getting the same scrutiny.

"Why is that Hillary Clintons family and Dems dealings with Russia are not looked at, but my non-dealings are?" he asked.

"They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice," Trump wrote in his first tweet. "You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA."

The Twitter attacks came as Vice President Mike Pence hired a personal lawyer to represent him in the intensifying investigation. Pence's office confirmed he had retained Richard Cullen, a former Virginia attorney general and U.S. attorney for the Eastern District of Virginia, to assist "in responding to inquiries" from Mueller.

Mueller's investigation appeared to be reaching a broadening circle of current and former officials. The Washington Post reported Wednesday that the investigators were looking into possible obstruction of justice. The newspaper noted Mueller had requested interviews with CIA Director Dan Coats, National Security Agency chief Michael Rogers and Richard Ledgett, the former NSA deputy director. Recent news reports have suggested Trump sought all three officials' help in pressuring FBI Director James Comey to drop his investigation into former National Security Adviser Mike Flynn.

Comey testified last week that he also felt pressured to drop the Flynn probe. Comey said he believes Trump ultimately fired him "because of the Russia investigation."

Coats met behind closed doors for more than three hours Thursday with the Senate intelligence committee, which is conducting a separate investigation into Russian interference in the election.

Trump's Twitter response followed days of intensifying criticism of Mueller from some conservatives who have charged the former FBI director and his team with political bias and have claimed his relationship to Comey and an earlier meeting with Trump amount to conflicts of interest.

A close Trump associate said this week the president was considering firing Mueller, although the White House later denied it.

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Associated Press writer Mary Clare Jalonick contributed to this report.