WASHINGTON — Robert Mueller, the special counsel investigating possible ties between President Trump’s campaign and Russia’s government, may expand his inquiry to investigate the roles of the attorney general and deputy attorney general in the firing of FBI Director James Comey.
Deputy Attorney General Rod Rosenstein said he would step aside from any oversight of Mueller if he were to become a subject of Mueller’s investigation.
Mueller has already taken over a separate criminal inquiry involving former Trump campaign chairman Paul Manafort
The Justice Department’s criminal investigation into Manafort predated the 2016 election and the counterintelligence inquiry that in July began investigating possible collusion between Moscow and associates of Trump.
Manafort was forced to resign as Trump campaign chairman in August amid questions over his business dealings years ago in Ukraine.
The move to consolidate the matters, involving allegations of misuse of Ukrainian government funds, indicates that Mueller is assuming a broad mandate in his new role running the investigation.
No one familiar with the matter has been willing to publicly discuss the scope of Mueller’s investigation because it is just getting underway and revealing details could complicate its progress.
Rosenstein acknowledged that Mueller could expand his inquiry to include the roles of Attorney General Jeff Session and Rosenstein in the decision to fire Comey, who was investigating the Trump campaign.
Rosenstein, who appointed Mueller as special counsel, 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’s pledge not to become involved in the Russia case.
Rosenstein said Mueller’s investigation could expand to include examining Sessions’s role.
‘‘The order is pretty clear,’’ Rosenstein responded. ‘‘It gives him authority for the investigation and anything arising out of that investigation, and so Director Mueller will be responsible in the first instance for determining what he believes falls into that mandate.’’
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.
‘‘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 after Comey’s firing on May 9. Comey is expected to testify for the first time before the Senate intelligence committee on Thursday.
Mueller’s assignment covers the 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 inquiry.
It would extend to any allegations of perjury, witness intimidation or obstruction of justice uncovered during the course of the investigation.
Last month, House Democrats called for congressional investigations into whether Sessions violated his pledge to recuse himself from matters related to investigations into Trump associates. They also asked the Justice Department to investigate Sessions’s role in Comey’s firing and to lay out how that investigation would proceed.
A Democratic aide said Representative Elijah Cummings, a Maryland Democrat, asked Rosenstein about the matter during a briefing before House members. Rosenstein said he would get back to Cummings, but he has yet to respond, said the aide, who spoke on condition of anonymity to discuss the private interactions.
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 Associated Press 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.