Special Counsel Robert Mueller’s team of lawyers reportedly asked witnesses to hand over their phones for inspection of possible encrypted communications between individuals with ties to President Donald Trump.

“Since as early as April, Mueller’s team has been asking witnesses in the Russia probe to turn over phones for agents to examine private conversations on WhatsApp, Confide, Signal and Dust, according to the sources, who spoke on condition of anonymity. Fearing a subpoena, the witnesses have complied with the request and have given over their phones, the sources said,” CNBC reports.

Andrew Weissman, the Special Counsel’s top prosecutor, filed an 18-page motion Friday evening at the U.S. District Court for the District of Columbia, accusing former Trump campaign chair Paul Manafort and an associated of attempting to tamper with witnesses scheduled to testify in the probe. Weissman requested Judge Amy Berman Jackson on Monday to arrange a hearing to review evidence of Manafort violating the terms of his bail “in an effort to influence their testimony and to otherwise conceal evidence,” related to work undertaken by a public relations firm tied to Ukrainian president Viktor Yanukovyc.