"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump wrote in a tweet.
ABC News has reached out to the White House for comment.
A source close to the president's legal team said that the tweet was not actually confirming an investigation, but rather referencing a Washington Post story Wednesday that reported Special Counsel Robert Mueller had expanded his probe of Russian election meddling to include whether Trump obstructed justice.
Officials who served on Trump's transition team have been instructed to preserve all documents that could be pertinent to the Russia investigations conducted by Congress and the special counsel.
In May, Deputy Attorney General Rod Rosenstein appointed former FBI director Robert Mueller as special counsel to take over the FBI’s investigation into Russia’s meddling in the election.
Trump has repeatedly denied that he is under investigation. “No collusion, no obstruction,” he remarked at a news conference last Friday.
The Washington Post reported that the obstruction of justice inquiry stems from Trump's firing of Comey, who was overseeing the FBI probe. ABC News has not been able to independently confirm the report.
After the report, Trump tweeted Thursday, "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."
Then, in what could be seen as an attack against Rosenstein, Mueller, or Mueller's special counsel team members, Trump called the special counsel's probe into Russia the "single greatest WITCH HUNT in American political history - led by some very bad and conflicted people!"
Comey was fired by Trump May 9. In a letter to Comey informing him of his dismissal, Trump said he was acting on a recommendation from Rosenstein. A few days later, in a May 11 interview, Trump told NBC News he was thinking of the Russia investigation when he ultimately made the decision to dismiss Comey.
Although Trump appeared to be attacking Rosenstein today, Trump previously praised the man he nominated for deputy attorney general.
"He’s highly respected. Very good guy, very smart guy. The Democrats like him. The Republicans like him," Trump said of Rosenstein in that same interview with NBC News.
As for the gathering of documents, Kory Langhofer, a lawyer on the transition team and of the Arizona law firm Statecraft, said he sent a memo Thursday ordering officials to do so related to Russia, Ukraine, and former campaign advisers and officials including Michael Flynn, Paul Manafort, Carter Page, Rick Gates and Roger Stone.
His memo asks aides to save all foreign travel documents and any records of communications from the transition period. Not preserving the records, Langhofer's memo warns, could result in civil or criminal penalties.
The request for records preservation is not unusual or unexpected. Similar legal instruction was given to Trump's campaign officials.
Langhofer declined to offer any additional comment to ABC News.
ABC News’ Alexander Mallin, Benjamin Siegel and Chris Donovan contributed to this report.