Washington (CNN) Federal prosecutors have issued grand jury subpoenas to associates of former National Security Advisor Michael Flynn seeking business records, as part of the ongoing probe of Russian meddling in last year's election, according to people familiar with the matter. CNN learned of the subpoenas hours before President Donald Trump fired FBI director James Comey.

The subpoenas represent the first sign of a significant escalation of activity in the FBI's broader investigation begun last July into possible ties between Trump campaign associates and Russia.

The subpoenas issued in recent weeks by the US Attorney's Office in Alexandria, Virginia, were received by associates who worked with Flynn on contracts after he was forced out as director of the Defense Intelligence Agency in 2014, according to the people familiar with the investigation.

Robert Kelner, an attorney for Flynn, declined to comment. The US Attorney's Office in Alexandria, the Justice Department and the FBI also declined to comment.

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