Special Counsel Robert Mueller and his team want witnesses in the Russia investigation to hand over their personal phones, CNBC reported Wednesday, in order to find out whether they’re using encrypted messaging apps to talk to people close to President Donald Trump.

Unnamed sources told CNBC that Mueller’s team has sought to inspect conversations on WhatsApp, Signal, Dust, and Confide — all programs that aim to provide users with more secure messaging. Mueller reportedly started asking for people’s phones as early as April.

So far, witnesses have turned over their phones in order to avoid a subpoena, CNBC reported.

On Monday, Mueller asked a judge to revoke Paul Manafort’s bail, after he used an encrypted messaging app to try to reach out to a witness involved in Manafort’s federal tax and money laundering case, according to court records. Manafort allegedly messaged an intermediary, known only as “D1” in court papers, with the words “this is Paul.”

Mueller’s team says this was an attempt to tamper with witnesses.