The FBI began its obstruction of justice case against President Trump before the appointment of special counsel Robert Mueller, newly unsealed court documents show.

In a January 2018 hearing, a prosecutor on Mueller's team revealed the obstruction inquiry was prompted by memos written by former FBI Director James Comey that contained notes from his one-on-one meetings with Trump.

"Before the appointment of the Special Counsel on May 17th, the FBI had opened an investigation into obstruction of justice," Michael Dreeben, a counsel for the special counsel’s office, said. "That investigation entailed matters that were covered in the Comey memoranda, which explored and recorded Mr. Comey’s recollections of meetings, including one-on-one meetings with the President of the United States. In those meetings, events occurred that led the FBI to conclude that an investigation was appropriate under its authority to consider matters such as obstruction of justice."

"In this instance, a person whose conduct is within the scope of the investigation is the President of the United States," Dreeben added. "The Comey memoranda record Mr. Comey’s recollections of one-on-one interactions with the President of the United States."

Comey detailed in his memos face-to-face conversations and phone calls he had with Trump earlier in 2017. In one conversation, Comey wrote Trump pressed him for a pledge of loyalty and, in another, Trump asked Comey to go easy on former White House national security adviser Michael Flynn, who has since pleaded guilty to lying to FBI investigators.

Comey was fired by Trump on May 9, 2017, and eight days later, Deputy Attorney General Rod Rosenstein appointed Mueller to take charge of an investigation into Russian interference in the 2016 election and possible links between the Trump campaign and the Kremlin. In a February interview with CBS's "60 Minutes," Andrew McCabe, who was acting FBI director for about a month, said he was the one to order the obstruction inquiry after Comey's ouster.

Before he was fired, Comey assured Trump in private that he was not under investigation, but officials said that changed days after Trump fired Comey, according to a Washington Post report in June 2018 that first notified the public of an obstruction inquiry. Rosenstein also repeatedly assured Trump he was not a "target" of the investigation.

Despite these assurances, in his final 448-page report, Mueller details 10 instances in which Trump might have obstructed justice, but declines to say whether Trump committed a crime, citing a Justice Department guideline that sitting presidents cannot be indicted. Although Attorney General William Barr said he and Rosenstein determined that there is insufficient evidence to prove an obstruction crime, Democrats argue that Mueller's report leaves it up to Congress to investigate and decide.

The hearing in January 2018 was part of a Freedom of Information Act lawsuit brought on by CNN and other media organizations over access to the Comey memos, which were released in redacted form three months later in April. The transcript was released Tuesday on a court order.

Dreeben argued against the release of Comey's memos due to concern that Trump and other witnesses might alter their stories depending on what they saw in the memos. Mueller's team was seeking an interview with the president himself at the time.

Dreeben also laid out Mueller's mandate, which included looking into possible obstruction.

"As background, on May 17, 2017, the Acting Attorney General appointed, as Special Counsel, Robert Mueller to conduct an investigation that was defined in the Acting Attorney General’s order," Dreeben said. "The investigation entailed exploring the matters that former FBI Director Comes had detailed in a hearing on March 20th in the House Committee appearance that he made as well as matters that arose or may arise from that investigation and matters that are covered by 28 CFR 600.4. That section of the Special Counsel regulations authorizes the Special Counsel to look into matters that would interfere with the investigation being conducted, including obstruction of justice."