President Donald Trump Alex Wong/Getty Images Special counsel Robert Mueller is probing what, and how much, President Donald Trump knew about the Russian government-backed effort to hack the Democratic National Committee, and the subsequent release of stolen emails at the height of the 2016 campaign.

Mueller is also looking into Trump's relationship with Roger Stone, a longtime Republican strategist who was in touch with WikiLeaks, its founder Julian Assange, and the Russian hacker Guccifer 2.0 during the election.

The revelations indicate Mueller is zeroing in on what appears to be a growing timeline of Trump associates' communications with Russia-linked actors during the final months of the campaign. Sign up for the latest Russia investigation updates here.

The special counsel Robert Mueller is asking witnesses in the Russia investigation whether President Donald Trump had prior knowledge about Russia's plans to hack the Democratic National Committee, whether he was involved in coordinating the release of stolen emails, and why he endorsed Russia-friendly policy positions during the campaign, NBC News reported on Wednesday.

Mueller's team has also asked about longtime Republican operative and Trump confidant Roger Stone's communications with Julian Assange, the founder of the radical pro-transparency group WikiLeaks.

Mueller is tasked with overseeing the FBI's investigation into Russia's interference in the 2016 election and whether members of the Trump campaign colluded with Moscow to tilt the race in his favor. Russia's hack of the DNC and WikiLeaks' involvement in disseminating the stolen emails make up a significant thread in the investigation.

In particular, NBC News reported, prosecutors are zeroing in on Trump's public appeal for Russia to recover then-Democratic candidate Hillary Clinton's deleted emails during a July 2016 press conference.

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said at the time.

Investigators are also reportedly delving into Trump's decades-long relationship with Stone, as well as Stone's contacts with Assange and WikiLeaks.

Stone told the House Intelligence Committee last September that he had never "said or written that I had any direct communication with Julian Assange and have always clarified in numerous interviews and speeches that my communication with WikiLeaks was through the aforementioned journalist."

Stone was referring to radio host Randy Credico, who Stone said acted as an intermediary between himself and Assange.

A timeline emerges

Trump's eldest son, Donald Trump Jr., was in touch with WikiLeaks several times between September 2016 and July 2017, The Atlantic reported last year.

WikiLeaks first contacted Trump Jr. via a private, direct message on Twitter on September 20, 2016 to tell him about a PAC-run anti-Trump website, called putintrump.org, and asked him if he had "any comments" on who was behind it.

WikiLeaks also told Trump Jr. that it had "guessed the password" to the anti-Trump website, and told him it was "putintrump."

Trump Jr. replied, "Off the record I don't know who that is, but I'll ask around. Thanks."

On the day he received that message, Trump Jr. emailed high-ranking campaign officials, including Steve Bannon, Kellyanne Conway, Brad Parscale, and Jared Kushner, informing them that WikiLeaks had contacted him.

Kushner also forwarded Trump Jr.'s email to Hope Hicks, who was a spokesperson for the Trump campaign. She is now the director of communications at the White House, though she has said she would resign in the coming weeks.

Trump Jr.'s communications with WikiLeaks didn't emerge until last year, but Stone has long drawn scrutiny, particularly after he sent out a series of tweets in 2016 that raised questions about whether he knew in advance that WikiLeaks was planning on publishing Clinton campaign manager John Podesta's hacked emails.

"Wednesday @HillaryClinton is done," Stone tweeted on October 1, 2016.

"I have total confidence that @wikileaks and my hero Julian Assange will educate the American people soon #LockHerUp," he tweeted two days later.

While WikiLeaks describes itself as a non-partisan transparency organization, it has been criticized for its apparently pro-Russia stance, particularly during the 2016 US election.

In addition to WikiLeaks and Assange, Stone was also in touch with Guccifer 2.0, a hacker said to be a front for Russian military intelligence.

Meanwhile, following its initial message to Trump Jr. on September 20, WikiLeaks touched base with him again on October 3 and told him it would be "great" if the campaign pushed a story about Hillary Clinton published by "True Pundit," a conservative-leaning outlet that's been known to spread junk news. According to the story, Clinton said she wanted to "just drone" WikiLeaks founder Julian Assange.

Trump Jr. replied that he had already shared the story earlier that day, adding, "It's amazing what she can get away with."

He also asked WikiLeaks about an upcoming document dump that Stone had tweeted about a day earlier.

Donald Trump Jr. (L) and Donald Trump (R). Saul Loeb-Pool/Getty Images

WikiLeaks did not respond to Trump Jr.'s question, and it ultimately published the first batch of hacked emails belonging to Podesta on Friday, October 7.

At a campaign rally three days later, Trump said he loved WikiLeaks. "It's amazing how nothing is secret today when you talk about the internet," he told the crowd.

He also tweeted about WikiLeaks on October 11, writing, "I hope people are looking at the disgraceful behavior of Hillary Clinton as exposed by WikiLeaks. She is unfit to run."

WikiLeaks then reached out to Trump Jr. on October 12, telling him that it was "great" to see him and Trump "talking about our publications." It also "strongly" suggested that Trump tweet out the link wlsearch.tk — which he did, two days later — claiming the site would help people search through the hacked documents. WikiLeaks also told Trump Jr. it had just released another batch of Podesta's emails.

An hour later, Trump tweeted: "Very little pick-up by the dishonest media of incredible information provided by WikiLeaks. So dishonest! Rigged system!"

"This WikiLeaks stuff is unbelievable," Trump said the same day, at a campaign rally in Florida. "It tells you the inner heart, you gotta read it."

That day, Stone admitted to having "back-channel communications" with Assange because they had a "good mutual friend."

"That friend travels back and forth from the United States to London and we talk," Stone told CBS' Miami affiliate. "I had dinner with him last Monday."

Trump again praised WikiLeaks on October 13, saying at a rally in Ohio that the content the group was pushing was "amazing."

The Atlantic reported Tuesday that Stone was also in direct contact with the group that day, less than a month before the election.

"Since I was all over national TV, cable and print defending wikileaks and assange against the claim that you are Russian agents and debunking the false charges of sexual assault as trumped up bs you may want to reexamine the strategy of attacking me," Stone reportedly wrote.

"We appreciate that," WikiLeaks replied. "However, the false claims of association are being used by the democrats to undermine the impact of our publications. Don’t go there if you don’t want us to correct you."

Two days later, on October 15, Stone reportedly wrote back: "Ha! The more you 'correct' me the more people think you’re lying. Your operation leaks like a sieve. You need to figure out who your friends are."

President-elect Donald Trump arrives to speak to a "USA Thank You" tour event, Thursday, Dec. 1, 2016, in Cincinnati. AP Photo/Evan Vucci

On November 9, 2016 — the day after Trump won the election — WikiLeaks replied, "Happy? We are now more free to communicate."

The group later sent another message, saying, "FYI, while we continue to be unhappy about false 'back channel' claims, today CNN deliberately broke our off the record comments."

CNN said the comments in question were not off-the-record because it made no such prior agreement with WikiLeaks before the conversation.

Stone said in a statement to NBC News that he had "no advance knowledge of the content or source of information published by WikiLeaks" and added that he had not been interviewed by Mueller's team.

"I never discussed WikiLeaks, Assange or the Hillary disclosures with candidate Trump, before during or after the election," he told the outlet. "I have no idea what he knew about them, from who or when. I have never met Assange."

Stone's relationship with Trump has also been of particular interest to investigators.

One witness interviewed by Mueller's team told NBC News that investigators asked about what Stone's interactions with Trump were like once he ended his tenure as a Trump campaign adviser in August 2015.

"How often did they talk? Who really fired him? Was he really fired?" the witness said, describing the questions they were asked.