Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s team has asked witnesses in its probe to turn over their personal phones for examination of encrypted messaging apps, CNBC reported Thursday.

Sources told the news outlet that Mueller’s team has been interested in conversations that took place among Trump associates on WhatsApp, Confide, Signal and Dust, which allow users to communicate using encryption.

At least some witnesses have reportedly already complied with the request.

ADVERTISEMENT

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

The report comes two days after Mueller's team alleged in a court filing that former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE attempted to tamper with a potential witness in his upcoming trial, in part using encrypted messaging apps.

Mueller's team said Manafort and a longtime associate linked to Russian intelligence attempted to contact via phone call, text and encrypted messages two people from The Hapsburg Group, which Manafort had worked with to lobby for Ukrainian interests.

The filing said Manafort's attempts at contact were “in an effort to influence their testimony and to otherwise conceal evidence,” and the probe into the matter was still ongoing.

Mueller's team asked the court to revoke or revise Manafort's release conditions, including his bond and house arrest.

In addition to Manafort, Mueller's probe into Russian meddling in the 2016 presidential election has thus far led to three other former Trump associates being indicted or pleading guilty. He also filed charges against 13 Russian nationals.

Manafort has pleaded not guilty on all charges.

The president has frequently decried the Mueller probe as a "hoax" and a "witch hunt." On Monday, he declared the special counsel is "unconstitutional," though he did not provide support for his claim.