WASHINGTON — Special counsel Robert Mueller is nearing the end of his historic investigation into Russian election interference and is expected to submit a confidential report to the attorney general as early as mid-February, government officials and others familiar with the situation tell NBC News.

"They clearly are tying up loose ends," said a lawyer who has been in contact with the Mueller team.

The sources either did not know or would not say whether Mueller has answered the fundamental question he was hired to investigate: Whether Trump or anyone around him conspired with the Russian intelligence operations to help his campaign.

Mueller has not made public any evidence proving such a conspiracy, though he has rebutted in court filings the president's assertion that neither he nor any of his top aides had met or talked with Russians during the 2016 race. They did, according to Mueller; and, in the case of his lawyer's negotiations over a Trump Tower in Moscow, Trump knew about it, court filings say.

Mueller has also examined the question whether the president obstructed justice, and is expected to address that matter in his report. Whether the special counsel will accuse the president of wrongdoing on that score is unclear.

Paul Manafort exits the E. Barrett Prettyman Federal Courthouse on Feb. 28, 2018 in Washington. Drew Angerer / Getty Images file

Mueller was appointed in May 2017 in the wake of Trump's decision to fire the FBI director, James Comey. He inherited an FBI investigation that had been launched in July 2016, when intelligence agencies saw indications that people around Trump might be trying to help a Russian effort to boost the Trump presidential candidacy.

He has charged 33 people and convicted three senior Trump associates — former campaign chairman Paul Manafort, former National Security Adviser Mike Flynn and lawyer Michael Cohen — who have cooperated with him to varying degrees.

One sign that Mueller is close to finishing, legal experts say, is that he has moved forward with the sentencing of those men — particularly Flynn, who he credited with substantial cooperation, much of which remains secret. Flynn's lawyers agreed to postpone his sentencing this week after it became clear the judge was considering imposing a prison sentence despite Mueller's recommendation of probation.

Generally, prosecutors prefer to delay the sentencing of cooperating witnesses until the case in which they are helping is over, to retain leverage over them and secure their testimony in court.

Mueller's spokesman, Peter Carr, declined to comment. The president's lawyer, Jay Sekulow, said, "We don't discuss conversations we've had or have not had with the office of special counsel."

Defense lawyers in the case have been talking among themselves about their belief that the investigation is coming to an end, two of them said.

Former White House National Security Adviser Michael Flynn leaves a sentencing hearing in U.S. District Court on Dec. 18, 2018. Chip Somodevilla / Getty Images

The sources who spoke to NBC News warn that a few major outstanding matters could complicate Mueller's endgame. One is Mueller's desire to interview the president about all aspects of his investigation, including obstruction of justice matters about which the president has refused to answer questions.

If Mueller moved to subpoena the president, that could spark months of litigation that could delay his report. A source familiar with the matter says Acting Attorney General Matthew Whitaker would have to approve any such subpoena.

Vice President Pence has not been interviewed by the special counsel and has not appeared before his grand jury, according to a person familiar with his status in the investigation. Pence turned over emails to Mueller covering the presidential transition, the person said.

Whitaker, who made a series of disparaging comments about the Mueller probe as a television commentator, has not recused himself from supervising the investigation, NBC News reported Thursday.

It's not clear whether the president's son, Donald Trump Jr., has been interviewed by Mueller. The president's son-in-law, Jared Kushner, spent seven hours answering the special counsel's questions in May, according to his lawyer, Abbe Lowell.

Michael Cohen exits the courthouse after his sentencing in New York on Dec. 12, 2018. Shannon Stapleton / Reuters

Another piece of unfinished business involves Trump associates Jerome Corsi and Roger Stone. Corsi has said he expects to be indicted for lying to investigators, which he denies, while Stone has told supporters, "Robert Mueller is coming for me," without offering specifics. The House Intelligence Committee voted Thursday to release a transcript of its interview with Stone at Mueller's request, a sign that that the special counsel could be moving to charge him. Corsi and Stone say they have done nothing wrong.

Mueller's report is not expected to address the separate investigation by New York federal prosecutors in which Cohen has implicated the president in a campaign finance felony. That investigation appears to be continuing, and Trump also will still have to contend with inquiries by the New York Attorney General and Congress.

U.S. officials familiar with the matter say the Justice Department and Congress have been planning for the delivery of a report by Mueller, on the assumption that at least some part of it would be made public. That would not be an easy process, given the extent to which the investigation has relied on classified information and grand jury testimony that is secret by law.

A Vietnam combat veteran who led the FBI after the 9/11 attacks, Mueller is not operating under the Independent Counsel statute that governed Kenneth Starr, whose salacious and controversial report about Bill Clinton's affair with a White House intern was delivered to Congress and the public at the same time in 1998, causing a sensation.

In Mueller's case, the acting attorney general, Whitaker, would receive the report and would have to decide what do to with it.

The regulations governing the special counsel say the special counsel "shall provide the Attorney General with a confidential report explaining the prosecution or declination decisions reached by the Special Counsel."

The Justice Department is likely to make some aspect of the report public, sources say. House Democrats, who will have subpoena power as of January, have said they will do everything they can to make sure it sees the light of day.