President Trump's attorney Michael Cohen departs after a closed-door meeting with the Senate Intelligence Committee in September. (Andrew Harnik/AP)

Special counsel Robert S. Mueller III has requested documents and interviewed witnesses about incidents involving Michael Cohen, the longtime lawyer for President Trump whose wide-ranging portfolio has given him a unique vantage point into Trump’s business, campaign and political activities.

There is no indication that Cohen is a subject or target of the investigation into Russian interference in the 2016 election. But the scrutiny of his interactions is another sign of the far-reaching nature of the special-counsel probe, which is examining members of the president’s inner circle and aspects of Trump’s past business outreach to Russia.

As one of Trump’s closest advisers, Cohen played a role in at least two episodes involving Russian interests that have drawn Mueller’s attention, according to several people familiar with document subpoenas and witness interviews.

One area of focus has been negotiations Cohen undertook during the campaign to help the Trump Organization build a tower in Moscow. Cohen brought Trump a letter of intent in October 2015 from a Russian developer to build a Moscow project. Later, he sent an email to Russian President Vladi­mir Putin’s chief spokesman seeking help to advance the stalled project. He said he did not recall receiving a response.

Another area that Mueller’s team has explored is a Russia-friendly peace proposal for Ukraine that was delivered to Cohen by a Ukrainian lawmaker one week after Trump took office, the people said.

Cohen is also among nine Trump associates whose communications with former Trump aide Sam Nunberg are being sought by the special counsel, according to a grand jury subpoena sent to Nunberg last week.

Cohen is the only individual on the list who never worked for Trump’s campaign or the White House — and the only one still working for the president.

Stephen Ryan, an attorney for Cohen, rejected the notion that his client was under particular scrutiny by Mueller.

“Unsourced innuendo like this succeeds only because the leakers know the Special Counsel will not respond to set the record straight,” he said in a statement.

A spokesman for the special counsel’s office declined to comment.

Known for his combative style and fierce loyalty to Trump, Cohen served for a decade as a top lawyer at the Trump Organization, tangling with reporters and Trump’s business competitors on behalf of the celebrity real estate mogul.

He never formally joined Trump’s campaign but was in close contact with his longtime boss from his Trump Tower office throughout the 2016 race and presidential transition.

Cohen’s aggressive tactics recently came into public view when he acknowledged he facilitated a $130,000 payment in October 2016 to an adult-film actress who claimed to have had a sexual encounter with Trump.

Cohen left the Trump Organization in January 2017, around the time of Trump’s inauguration, and since then has served as a personal attorney to the president.

Despite having no formal role in the administration, Cohen was a frequent White House visitor in the months after Trump took office, often dropping by the Oval Office without an appointment to visit with the president, according to three current and former administration officials.

Cohen’s access worried some senior aides in the West Wing, who felt he tended to bring out the president’s scorched-earth tendencies. Cohen told others that Trump was fond of him and expressed his desire to have him in the White House.

Cohen has been seen far less in the White House during the past six months but remains in regular contact with the president, according to people familiar with his role.

It is unclear how aggressively Mueller’s prosecutors have been probing issues involving Cohen, but they have periodically sought information related to the longtime Trump lawyer during the past several months, including in recent weeks, according to people familiar with the special counsel’s investigation.

Cohen figured in two key episodes related to the former Soviet Union during and immediately after the campaign, including discussions about the never-realized Trump Tower Moscow.

Cohen has said he worked on the deal with Felix Sater, a real estate developer who helped build a number of Trump-branded properties, including Trump SoHo in New York, and had tried to help Trump build in Moscow a decade earlier.

The most recent effort to launch a Trump-branded development in Moscow began in the fall of 2015, at the same time Trump was competing for the GOP presidential nomination.

In January 2016, Cohen emailed Dmitry Peskov, Putin’s spokesman, to ask for help advancing the project, according to documents submitted to congressional investigators.

Cohen said in an August statement that he did not recall receiving a response. He said that the plan was abandoned in January 2016 “for business reasons” when government permission was not secured and that the matter was “not related in any way to Mr. Trump’s presidential campaign.”

Mueller has requested documents related to the project, and his prosecutors have asked witnesses about it, according to people familiar with the probe.

Sater and Cohen also figure in the Ukrainian peace proposal, another episode that has been reviewed by special-counsel investigators, the people said.

Robert S. Wolf, an attorney for Sater, said he was not aware of any recent interviews by the special counsel regarding the episodes involving Sater.

“Mr. Sater’s description of these events has never changed one iota,” he added.

In a statement, Sater said that “the only purpose for the Ukrainian peace proposal was to save lives, which is a noble pursuit.”

Sater organized a Jan. 27, 2017, meeting at the Loews Regency hotel in New York during which Ukrainian lawmaker Andrii V. Artemenko gave Cohen the proposal, according to interviews last year with the three participants.

The back-channel proposal offered a pathway for resolving the Ukrainian dispute that could have led to the lifting of U.S. sanctions on Russia and given Putin a prize he has long sought — undisputed control over Crimea, territory that Russia seized in 2014.

The plan was presented to Cohen a few weeks after then-national security adviser Michael Flynn was heard in an intercepted phone call telling the Russian ambassador that the Trump administration would roll back sanctions imposed by the Obama administration intended to punish Moscow for interfering in the 2016 election.

All three men who participated in the January meeting have said that the initiative was driven solely by their joint desire for peace in war-torn Ukraine and that nothing came of the proposal.

But Cohen, Sater and Artemenko — who at the time served in the Ukrainian parliament — have offered conflicting accounts of key aspects of their interactions, including whether the proposal reached the White House, as Artemenko had intended.

The New York Times, which first reported the meeting last year, said Cohen told the newspaper he left the plan in Flynn’s White House office about a week before Flynn resigned.

In an interview last year, Artemenko said Cohen suggested that Flynn would be the correct person at the White House to receive the plan, but he said Cohen made no promises to deliver it. Nevertheless, Artemenko said he was told later by Sater that Cohen had gotten the plan to Flynn.

However, Sater told The Washington Post last year that he understood Cohen intended to give the plan to Flynn but that the national security adviser resignation’s interceded.

In interviews last year with The Post, Cohen called the Times story “fake news.” He denied that he gave the proposal to Flynn or that he had ever said he had done so.

Instead, Cohen told The Post he threw away the unopened envelope containing the plan in a trash can at his New York apartment.

“I never looked at it,” Cohen said. “I never turned it over to anyone.”

The men also differed about the extent of Russian involvement in the proposal. Cohen told The Post that Artemenko had announced during their brief meeting that he had devised the proposal in consultation with the Russian government.

“He said Russia was on board — the Russian government,” Cohen said.

Artemenko, who had served in the Ukrainian parliament since 2014, told The Post last year that he spoke to no Russian officials and that his proposal was the product of consultations among Ukrainians, including other lawmakers.

“The Russians are not involved in this conversation,” he said in a phone interview last year. “I’m a guy who was born here, who really wants to help. . . . My intention is very simple — complete cessation of all hostility.” Reached last week, he declined to comment.

A spokesman for the Kremlin has said that Russia had no advance notice of the plan and did not support it.