A newly unsealed court document reveals the FBI opened an obstruction of justice case against President Trump before Special Counsel Robert Mueller was appointed to investigate Russia’s interference in the 2016 election.

Trump claimed earlier this year that he was assured by Deputy Attorney General Rod Rosenstein he was not a target or subject of Mueller's probe.

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But the final Mueller report showed his investigators looked closely at the question of whether Trump obstructed justice, while not reaching a conclusion. And according to the court document released this week, that avenue of investigation was being pursued before Mueller even took over. Michael Dreeben, an attorney for the special counsel’s office, said in a January 2018 hearing in federal court that in 2017, “Before the appointment of the Special Counsel on May 17th, the FBI had opened up an investigation into obstruction of justice.”

The 2018 hearing concerned an unsuccessful lawsuit from CNN seeking copies of former FBI Director James Comey’s memos about his interactions with Trump. CNN published a transcript of that hearing Tuesday after the Justice Department, under a court order, was forced to provide the transcript to the cable network.

CNN previously cited unnamed sources in saying then-acting FBI Director Andrew McCabe opened the obstruction investigation after Comey was fired and before Mueller was appointed. McCabe later said in an interview he launched the probe at that point. The court filing provides additional confirmation of that timeline.

During the court hearing, meanwhile, the special counsel’s office argued against releasing the Comey memos, saying they would interfere with the investigation.

Mueller was appointed by Rosenstein in May 2017 to investigate any links or coordination between the Russian government and individuals associated with the Trump campaign, as well any other matters that arose from the investigation.

Mueller also went on to investigate whether Trump obstructed justice with his comments and actions related to the probe, including through his firing of Comey, his comments about the investigation into then-National Security Adviser Mike Flynn and his ousting of Attorney General Jeff Sessions.

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In his report released last month, Mueller stated his investigators found no evidence that Trump’s campaign illegally coordinated with Russia. But Mueller did not reach a conclusion on whether Trump obstructed justice, though it said Trump told White House counsel Don McGahn to seek Mueller's removal – something he never did.

Democrats on Capitol Hill have zeroed in on the obstruction allegations listed in the report, vowing to investigate the details while not ruling out impeachment proceedings.

Republicans have pushed back, saying the president could not have obstructed justice since Mueller found no underlying crime by Trump associates of coordinating with the Russians in the 2016 election.