Special Counsel Robert Mueller's investigative team has asked the Department of Justice to turn over a broad array of documents as part of its ongoing investigation into whether the Trump campaign colluded with Russian operatives during last year's presidential election.

A source briefed on the request told ABC News that Mueller is particularly interested in documents related to President Donald Trump's dismissal of former FBI Director James Comey and Attorney General Jeff Sessions' decision to recuse himself from the Russia investigation earlier this year. Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein were integral to Comey's removal.

Mueller issued the directive––his first records request to the Justice Department––last month, and it means he is now demanding documents from the very department overseeing his investigation. The directive is the latest indication that at least some aspects of Mueller’s probe focus on the president’s behavior in office, and whether he improperly attempted to wield influence over the investigation. Trump has been very vocal about his disdain for the probe, repeatedly undermining the investigation––which he claims is a political ploy orchestrated by Democrat lawmakers––via his personal Twitter account.

The news prompted several prominent figures, including Representative Ted Lieu (D-HI), to weigh in on what this stage of the investigation could entail.

Former federal prosecutor Renato Mariotti believes the request for documents related to Sessions' recusal is "interesting" because "Earlier reports indicated that Trump exploded when he found out about his recusal. That could be evidence of his state of mind because it is a highly unusual reaction to a recusal decision." He also notes that "proving Trump had “corrupt” intent when he fired Comey is the big hurdle that Mueller must clear."

A New York Times report in May revealed that Trump asked Comey to halt the criminal investigation into former national security adviser Michael Flynn, who provided false information about his communications with the Russian government. Flynn resigned in disgrace after the news of his conversations with Sergey Kislyak, who was then the Russian ambassador to the United States, became public.

I hope you can see your way clear to letting this go, to letting Flynn go,” Trump told Comey, according to a memo Comey wrote immediately after the meeting, which took place the day after Flynn resigned. “He is a good guy. I hope you can let this go.”

Comey wrote the memo detailing his conversation with Trump as part of a paper trail documenting the president’s “improper” efforts to impede the continuing investigation. Trump fired Comey on May 9, an action which, many legal experts say, constitutes grounds for an investigation of Trump for possible obstruction of justice.

Last week, Mueller issued a subpoena to the Trump campaign for more documents related to its Russia ties. Mueller “had previously received the information that had already been handed over to Congress, but investigators felt there were things they didn’t see. The new subpoena seeks more records based on various search terms,” according to CNN. The Wall Street Journal was the first to break the story.

One source described it as a “cleanup” subpoena––and the campaign is complying. Another source called the subpoena “bookkeeping” and “largely perfunctory,” adding that much of the documents the special counsel receives will be will be “duplicative” of ones he already has. Indeed, The Journal‘s reporters note, “Sending a subpoena to an entity that says it has been cooperating with document requests isn’t unusual in cases in which prosecutors have some concern that their demands aren’t being met promptly or aren’t being entirely fulfilled.”

Last month, federal authorities charged former Trump campaign chairman Paul Manafort and his longtime business partner Rick Gates with 12 counts of “conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading FARA statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts.”

Manafort had expected charges since July, when FBI agents working for Mueller raided his Alexandria home in late July. Prosecutors had also warned Manafort that they planned to indict him, according to two sources familiar with the investigation. Investigators had earlier wiretapped Manafort, but dropped their investigation, citing lack of evidence. They later reopened the case because of intercepted communications between Manafort and suspected Russian operatives, and among the Russians themselves.

A spokesman for Mueller declined to speak to reporters about the latest developments in the Russia probe. A spokeswoman for the Justice Department also declined to comment.