Special counsel Robert Mueller has started writing his report of the investigation into Russian interference in the 2016 presidential campaign, CNN reported Thursday.

It is unclear when Mueller started writing the report, which would be delivered to the Justice Department. The special counsel is waiting on President Donald Trump’s legal team to submit responses to a series of questions provided by prosecutors within recent months.

Trump’s lawyers expect to submit the responses by the end of the month, according to CNN’s sources. Trump has already begun reviewing his attorneys’ responses to those questions.

Mueller, a former FBI director, has been on the investigation for nearly 18 months. Deputy Attorney General Rod Rosenstein appointed him to the position on May 17, 2017, a week after the firing of James Comey as FBI director. (RELATED: Mueller Expected To Make Moves After Midterms — Here’s What He Could Do)

Mueller initially continued the FBI’s investigation into possible collusion between the Trump campaign and Russian government that was started on July 31, 2016. The special counsel’s inquiry has since expanded to investigating whether Trump obstructed justice by firing Comey.

Trump and his attorneys have insisted that he had executive authority to fire Comey for any reason he saw fit. Trump lawyers Rudy Giuliani and Jay Sekulow have reportedly battled with the special counsel over what questions would be posed to Trump. The attorneys have said that Trump is willing to answer questions about collusion, which he denies occurred, but not obstruction.

A report from Mueller could mark the end of the special counsel’s probe. Regulations governing special counsel investigations state that, “At the conclusion of the Special Counsel’s work, he or she shall provide the Attorney General with a confidential report explaining the prosecution or declination decisions reached by the Special Counsel.”

The Justice Department can choose whether or not to release the report to the public.

If the report is submitted soon, those decisions will fall to Matthew Whitaker, the acting attorney general. Whitaker took over control of the DOJ on Wednesday after Trump forced Jeff Sessions to resign.

Trump critics expressed concern over Whitaker because of his past criticism of the Mueller probe. Whitaker, who served as Sessions’ chief of staff, led a nonprofit group during the 2016 presidential campaign that submitted several ethics violation complaints against former Secretary of State Hillary Clinton and Democrats.

A spokesman for the special counsel declined comment.

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