Paul Manafort Chip Somodevilla/Getty Images WASHINGTON (AP) — The special counsel investigating possible ties between President Donald Trump's campaign and Russia's government has taken over a separate criminal probe involving former Trump campaign chairman Paul Manafort, and may expand his inquiry to investigate the roles of the attorney general and deputy attorney general in the firing of FBI Director James Comey, The Associated Press has learned.

The Justice Department's criminal investigation into Manafort, who was forced to resign as Trump campaign chairman in August amid questions over his business dealings years ago in Ukraine, predated the 2016 election and the counterintelligence probe that in July began investigating possible collusion between Moscow and associates of Trump.

The move to consolidate the matters, involving allegations of kleptocracy of Ukrainian government funds, indicates that Special Counsel Robert Mueller is assuming a broad mandate in his new role running the sensational investigation. The expansiveness of Mueller's investigation was described to the AP. No one familiar with the matter has been willing to discuss the scope of his investigation on the record because it is just getting underway and because revealing details could complicate its progress.

In an interview separately Friday with the AP, Deputy Attorney General Rod Rosenstein acknowledged that Mueller could expand his inquiry to include Attorney General Jeff Sessions' and Rosenstein's own roles in the decision to fire Comey, who was investigating the Trump campaign. Rosenstein, who appointed Mueller as special counsel to take over the investigation, wrote the memorandum intended to justify Trump's decision to fire Comey. Sessions met with Trump and Rosenstein to discuss Trump's decision to fire him despite Sessions' pledge not to become involved in the Russia case.

Rosenstein told the AP that if he were to become a subject of Mueller's investigation, he would recuse himself from any oversight of Mueller. Under Justice Department rules, Mueller is required to seek permission from Rosenstein to investigate additional matters other than ones already specified in the paperwork formally appointing Mueller.

"I've talked with Director Mueller about this," Rosenstein said. "He's going to make the appropriate decisions, and if anything that I did winds up being relevant to his investigation then, as Director Mueller and I discussed, if there's a need from me to recuse I will."

Peter Carr, a spokesman for Mueller, declined to comment on the scope of the investigation.

Mueller, who spent 12 years as FBI director and served under Republican and Democratic presidents, was appointed as special counsel following the May 9 firing of Comey, who is expected to testify for the first time next week before the Senate.

FBI Director Robert Mueller AP Images

Mueller's assignment, detailed in a one-page order signed by Rosenstein, covers the federal investigation into possible links or coordination between Russia and associates of the Trump campaign but also "any matters that arose or may arise directly" from the probe. It would also extend to any allegations of perjury, witness intimidation or obstruction of justice uncovered during the course of the investigation.

As Mueller's investigation begins, members of Congress are intensely interested in its direction and scope.

Campaign manager Kellyanne Conway and Paul Manafort of Republican presidential nominee Donald Trump's staff speak during a round table discussion on security at Trump Tower in the Manhattan borough of New York, U.S., August 17, 2016. REUTERS/Carlo Allegri The Justice Department began looking at Manafort's work in Ukraine around the beginning of 2014, as Ukraine's president, Viktor Yanukovych, was toppled amid protests of alleged corruption and Russian influence. Business records obtained by the AP show Manafort's political consulting firm began working as early as 2004 for clients that variously included a political boss in Yanukovych's party, a Ukrainian oligarch and Oleg Deripaska, a Russian businessman and longtime ally of Russian president Vladimir Putin.

A special counsel, by design, is constrained by the terms of his appointment to avoid boundless and perpetually open-ended investigations. In this case, though, Mueller's mandate appears fairly broad, said Samuel Buell, a former federal prosecutor and criminal law professor at Duke University.

"That investigation that's named in the appointment is already one that has, as far as we can tell, a number of tentacles and offshoots that involves conduct over a fairly lengthy period of time involving a lot of people," Buell said.

He said he did not expect Mueller to seek Rosenstein's approval each time he wants to subpoena another new witness or pursue a new Russia-related investigative thread. The more difficult question would involve any allegations separate and apart from Russia, he said.

"This gives him the authority to pull on all kinds of string and see where they lead him," Buell said. "As long as you're following a string that's connected to the string of Russian influence on the election — however that may have occurred, whoever that may have involved — would seem to fall within that appointment."

Manafort's work in Ukraine continued at least through the beginning of 2014, when Yanukovych's government was ousted amid protests of widespread corruption and his rejection of a European trade deal in favor of one with Moscow. As the AP reported last year, that work included covertly directing a lobbying campaign on behalf of Ukraine's pro-Russian Party of Regions in Washington. Following the AP's reporting on emails in which Manafort deputy Rick Gates was overseeing the work, two lobbying firms involved in the project registered as foreign agents. Manafort has not done so, and a spokesman for him has declined to say if he will.