Associates of former National Security Adviser Mike Flynn have reportedly been subpoenaed by federal prosecutors investigating Russia’s meddling in the presidential election.

CNN reported late Tuesday that the grand jury subpoenas represent “a significant escalation” of the probe into potential collusion between President Donald Trump’s campaign team and Russian officials.

Read:With Mike Flynn’s Russia ties, it was a matter of following the money trail

Flynn, who was ousted in February after lying about his contacts with Russia’s ambassador to the U.S., is being investigated for potentially improper payments from the Russian and Turkish governments. Flynn may have broken the law by not properly disclosing a $45,000 payment for a 2015 appearance in Moscow for the pro-Putin TV network Russia Today. In March, Flynn’s lobbying company registered as a foreign agent representing Turkish interests.

The subpoenas appear to focus on Flynn’s business contacts after he was fired by the Obama administration in 2014, and before he joined Trump’s campaign team, CNN said.

CNN said it learned of the subpoenas hours before Trump fired FBI Director James Comey, whose department is leading the Russia probe.

On Monday, former acting Attorney General Sally Yates told a Senate committee that she warned the Trump White House in January that Flynn had misled them, and was at risk of being blackmailed by Russia. Flynn was not fired by Trump until 18 days later.