Special counsel Robert Mueller's investigators are asking witnesses to give them their personal phones so the devices can be inspected for encrypted messaging applications and possible communications between President Trump's associates, according to a report.

Mueller's team has been making the request of witnesses interviewed as part of the federal Russia probe since April, searching for apps such as WhatsApp, Confide, Signal, and Dust, per CNBC.

A spokesman for Mueller declined to respond to the news outlet's request for comment.

The report follows prosecutors for the special counsel's office accusing former Trump campaign chairman Paul Manafort of trying to tamper with potential witnesses using encrypted text messages sent in WhatsApp and Telegram ahead of his federal court trial in Washington, D.C.

Prosecutors allege in court documents filed Monday that Manafort "repeatedly contacted" two former employees in April in the hope of securing "materially false testimony" from them about the Hapsburg group, a collection of European leaders Manafort once incorporated in a lobbying strategy on behalf of Ukraine.

Manafort, 69, was indicted by Mueller — who took over the federal Russia investigation in May 2017 — in both Washington, D.C., and Virginia.

In Washington, Manafort has been charged with money laundering and lying about his lobbying work. In Virginia, Manafort has been charged with bank and tax fraud.

Manafort has pleaded not guilty in both cases.