Attorney Joe diGenova amplified a conspiracy theory about special counsel Robert Mueller during an interview Wednesday on Fox Business.

On Lou Dobbs Tonight, diGenova, a former U.S. attorney for the District of Columbia, referred to an interview Mueller had with President Trump in May 2017 about taking over as FBI director after James Comey was fired.

"Something very interesting that hasn't been written about occurred," diGenova told guest host Gregg Jarrett. "Bob Mueller went into that meeting with [Deputy Attorney General] Rod Rosenstein, and it was just three of them in the Oval Office, and Bob Mueller had a telephone, a cellphone," diGenova told Jarrett.

"He left that cellphone in Oval Office after the meeting. And it is now believed by many people that cellphone was used to broadcast the meeting between the president, Rod Rosenstein, and Bob Mueller, back to FBI headquarters, where they recorded the conversation with the president," diGenova said, before adding that Mueller likely left the phone there to "record what happened after they left."

Talk of Mueller leaving behind his cellphone in the Oval Office began with a book written by former FBI Deputy Director Andrew McCabe. He only mentions the phone off-hand and makes no assertions about clandestine surveillance.

"Mueller had gone in for an interview with Trump, and left his phone there, and then the phone had to be retrieved," McCabe wrote in The Threat: How the F.B.I. Protects America in the Age of Terror and Trump.

Speculation about the FBI recording the president has ensued on right-wing blogs and social media and now has been given a voice on cable television. DiGenova's claim was not contested by Jarrett.

DiGenova served as an independent counsel in the 1990s for a case on President Bill Clinton's passport before Clinton was elected. Last year, it was announced diGenova and his wife Victoria Toensing were joining Trump's legal team for the federal Russia investigation, but that plan was nixed within days. He has been highly critical of Mueller's Russia investigation, claiming that Trump had been "framed" by the Justice Department and the FBI.

Mueller is slated to testify about his investigation before two House panels next week. Jarrett, a Fox News legal analyst, has said a "very important" question will be whether Mueller should have accepted the special counsel job in the first place because of the interview he had about the FBI director position before he was appointed to head the Russia inquiry. Mueller served as FBI director from 2001 to 2013.