Former White House national security adviser Michael Flynn may be under investigation for his activities during his tenure at the Defense Intelligence Agency (DIA) under former President Obama, according to a letter from the agency sent to BuzzFeed News.

The letter, which came in response to a 2014 Freedom of Information Act request, denied a BuzzFeed reporter's request for Flynn's emails, job evaluations and other records from his time heading the DIA, saying that releasing the documents could "interfere with ongoing law enforcement investigative activities."

"Upon review of your request, I have determined that the release of potentially responsive records concerning LTG Flynn could reasonably be expected to interfere with on-going law enforcement investigative activities," it reads.

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Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE, who is conducting the criminal investigation into Russia's role in the 2016 election, has previously been reported to be looking into Flynn's activities since leaving the DIA in 2014. In particular, he is probing Flynn's financial ties to Russia and Turkey.

But according to BuzzFeed, the DIA's letter denying the records request suggests that Mueller is also looking into Flynn's activities while he served as the agency's director from 2012 to 2014.

The letter does not detail any potential investigative activities. But federal agencies are allowed to withhold requested documents from the public if they are compiled for law enforcement purposes.

“It certainly suggests that Flynn is being investigated not just for conduct that postdated his departure [from the DIA],” Stephen Vladeck, a law professor at the University of Texas, told BuzzFeed.

Flynn has emerged as a central figure in Mueller's investigation into Russia's election meddling. He resigned from his White House post in February – just 24 days into office – after it was revealed that he misled White House officials about his conversations with then-Russian Ambassador Sergey Kislyak in the month before President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE took office.

Since then, Flynn has faced scrutiny for his failure to previously disclose his financial ties to Russia and Turkey. He registered as a foreign agent with the Justice Department in March.

Reports last week suggested that Flynn may be cooperating with Mueller in his investigation, after his lawyers cut ties with attorneys representing Trump in the Russia probe.