Investigators for special counsel Robert Mueller have asked witnesses in their probe about President Trump's connection to Democratic emails that were hacked and leaked leading up to the 2016 election, NBC News reported Wednesday.

Mueller's team has reportedly focused on whether Trump was aware of plans for WikiLeaks to publish emails from the Democratic National Committee (DNC) and members of Hillary Clinton's campaign, or if Trump was involved in the emails' release.

Investigators have also honed in on Trump's relationship with Republican operative Roger Stone, an informal adviser during the 2016 campaign. They have reportedly asked witnesses about Stone's connections to WikiLeaks, and if he's met the site's founder, Julian Assange.

Stone told NBC in a statement that he has not been interviewed by the special counsel and "never discussed WikiLeaks, Assange or the Hillary disclosures with candidate Trump, before during or after the election."

The lines of questioning come as Mueller probes Moscow's meddling in the 2016 presidential election, including ties between Trump campaign associates and Russia.

In March 2016, Russians successfully hacked Clinton campaign chairman John Podesta's email account. WikiLeaks later released stolen emails from the DNC as well as emails from Podesta himself in the months leading up to the 2016 election.

Seizing on the controversy, Trump declared "I love WikiLeaks" at a campaign rally while he read some of the leaked messages.

According to NBC, investigators are also interested in Trump's comments on the campaign trail about emails then-opponent Clinton deleted from the private email server she used while secretary of State

"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said during the July 2016 press conference.

The White House later said Trump was joking when he made those comments.

Trump has repeatedly declared the federal Russia probe a "witch hunt" and has denied any collusion between his campaign and Russia.

Trump's former national security adviser Michael Flynn and former campaign associates George Papadopoulos and Richard Gates have pleaded guilty to charges brought as part of Mueller's investigation.

Former Trump campaign chairman Paul Manafort is also facing numerous charges for alleged financial crimes relating to his work as a political agent in Ukraine.

Mueller also filed charges earlier this month against 13 Russian nationals and three Russian organizations for alleged interference in the 2016 race.

Updated: 3:23 p.m.