The choice of Michael Flynn as Donald Trump's national security adviser reportedly prompted concerns from US intelligence over his links to Russia before he even accepted the role.
Flynn's "worrisome" behaviour included a history of contact with Russia and links to a woman who had access to Russian spy records, the Guardian revealed.
A number of sources told the newspaper that the choice of Flynn for the role led to discussions between US and UK intelligence prior to his appointment.
His time in the role was short-lived, with a leak about his Russian links and his subsequent deception of vice-president elect Mike Pence over it forcing the retired general to hand in his resignation from Trump's team.
In his resignation letter, Flynn said that he had "inadvertently briefed the vice-president elect and others with incomplete information regarding my phone calls with the Russian ambassador".
At the time, Trump suggested the leaks from the White House that led to Flynn's disgrace were an attempt to distract from Hillary Clinton's election loss. He also blamed the media for the way the issue had been reported.
"I think he's been treated very, very unfairly by the media — as I call it, the fake media, in many cases," Trump said in a press conference on Wednesday reported by The Hill.
"I think it's really a sad thing he was treated so badly."
Most recently, Flynn has offered to testify to officials investigating Russia's alleged involvement in the US election in exchange for immunity.
A lawyer for Flynn told Associated Press that he was in talks with congressional panels and seeking to avoid "unfair prosecution".
However, Flynn's offer was rejected and he was told the move was "wildly preliminary" and "not on the table" at this point, a senior congressional official told NBC.
The House Oversight Committee is looking at Flynn's activities prior to him accepting the NSA role, including contact he had with Russian officials and what security checks were carried out ahead of his appointment.