Federal prosecutors have issued grand jury subpoenas to associates of former White House national security adviser Michael Flynn amid the ongoing probe into alleged Russian election interference, CNN reported Tuesday.

The subpoenas, which CNN learned of just hours before President Donald Trump fired FBI director James Comey, were reportedly issued in recent weeks by the U.S. Attorney's Office in Alexandria, Virginia, to associates who worked with Flynn after he was forced out as director of the Defense Intelligence Agency in 2014.

Investigators are reportedly looking into possible wrongdoing in how Flynn handled disclosures about payments from foreign governments, including Russia and Turkey.  Former Acting Attorney General Sally Yates told a Senate judiciary subcommittee this week that she had warned the White House that Flynn was "compromised" and could be blackmailed over his ties to Russia.

CNN said the subpoenas signaled an escalation of the investigation against Flynn, but Just Security noted that this was not necessarily the case, "given the laundry list of potential charges he faces."

Flynn was forced to resign as Trump's national security adviser in February after revelations that he had lied to Vice President Mike Pence about conversations he'd had with Russian ambassador Sergey Kislyak. He has also been accused of possibly breaking the law by failing to disclose a $45,000 payment he received for making an appearance at an event in Moscow celebrating the state-run news outlet Russia Today, as well as a $530,000 payment from a Dutch company that has links to the Turkish government.

The subpoenas are part of a broader investigation against Flynn, which Comey testified last week was led by the Alexandria office and the U.S. Justice Department's National Security Division.