During his interviews with Special Counsel Robert Mueller's office, former National Security Adviser Michael Flynn told investigators that people with ties to the Trump administration and Congress contacted him in an attempt to interfere with the Russia probe, Mueller wrote in newly unredacted court papers released Thursday.

The messages could have "affected both his willingness to cooperate and the completeness of that cooperation," Mueller said, adding that "in some instances," his office was "unaware of the outreach until being alerted to it by the defendant." Flynn provided a recording of one of the voicemails he received, the filing said. In December 2017, Flynn pleaded guilty to lying to the FBI about his conversations with former Russian ambassador Sergey Kislyak before President Trump's inauguration. Flynn faces up to six months in prison, but due to his "substantial assistance," Mueller's office suggested he receive little to no time in prison.

In Mueller's report, he did not make a conclusion on whether Trump obstructed justice. The report also states that Trump's personal lawyer left a message for Flynn in November 2017, saying it "wouldn't surprise me if you've gone on to make a deal with ... the government," adding that if he shared any information implicating Trump, "then we've got a national security issue ... we need some kind of heads up."

Separately on Thursday, Judge Emmet Sullivan ordered federal prosecutors to file transcripts of the voicemail message left for Flynn and recordings of conversations Flynn had with Russian officials.