Special Counsel Robert Mueller has reportedly asked witnesses in the ongoing investigation of Russian meddling in the 2016 presidential election to hand in their phones to "inspect their encrypted messaging programs and potentially view conversations between associates linked to President Donald Trump."

On Monday, Mueller’s office formally accused Paul Manafort of felony witness tampering by using Telegram and WhatsApp, two well-known secure messaging apps. Manafort, who previously served as Trump’s campaign manager during the 2016 presidential race, pleaded not guilty to money laundering, among other charges. Authorities were able to obtain these messages by getting a warrant for Manafort’s iCloud account and also by getting unencrypted messages themselves from the recipients of some of those messages.

According to CNBC, since April, the Office of the Special Counsel has been looking at witnesses’ phones for evidence of the use of not only those two apps, but also Confide, Signal, and Dust, which operate in a similar fashion.

A spokesman for the Office of the Special Counsel would not comment on the CNBC report. "Thanks for reaching out," Peter Carr emailed Ars. "We are declining to comment."

Confide is at the center of an ongoing lawsuit involving former Missouri Governor Eric Greitens and his use of the ephemeral messaging app.

Both Confide and Signal were also reportedly used in the Trump White House before then-Press Secretary Sean Spicer banned them in February 2017, saying that the apps likely were not compatible with the Presidential Records Act.