Federal prosecutors have reportedly issued grand jury subpoenas as part of an investigation into alleged Russian hacking in the 2016 presidential election.
The summoning orders – reportedly demanding business records from associates of former National Security Adviser Michael Flynn – mark a significant step in the probe into the Trump campaign’s possible collusion with Russia.
The White House said Mr Comey was fired at the Justice Department’s recommendation. The US President said he had lost confidence in Mr Comey over his handling of an election-year email scandal involving then-Democratic presidential nominee, Hillary Clinton.
The subpoenas went to associates who worked with Mr Flynn when he was a private citizen, following his firing as director of the Defence Intelligence Agency in 2014, sources told CNN.
As part of their broader probe, prosecutors are reportedly investigating whether wrongdoing was committed by Lt Gen Flynn in his dealings with clients linked to foreign governments including Russia and Turkey.
Lt Gen Flynn later joined the Trump campaign but was fired as the White House national security adviser by Mr Trump in February after he failed to tell the truth about his contacts with the Russian ambassador to the US.
Lt Gen Flynn's attorney, Robert Kelner, did not respond immediately to requests for comment. A Justice Department spokeswoman said the department did not comment on the existence of ongoing investigations.
Russia has repeatedly denied meddling in the US election and the Trump administration has staunchly denied allegations of a conspiracy with Russia.
Mr Flynn has emerged at the centre of the months-long Russian contact scandal.
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