CNBC reveals investigators working with Special Counsel Robert Mueller are taking a close look at witnesses' use of encrypted messaging applications on their personal cellphones.
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.
While it's unclear what Mueller has discovered, if anything, through this new request, investigators seem to be convinced that the apps could be a key to exposing conversations that weren't previously disclosed to them.
On Monday, the government filed a motion accusing Paul Manafort of witness tampering, alleging the already indicted former Trump campaign chairman used an encrypted messaging program to contact two former associates and attempt to influence their testimony.
CNBC:
For evidence, Mueller's deputy listed two apps, WhatsApp and Telegram, that they say Manafort used to contact the witnesses in his case. The filing also says that those conversations were provided to Mueller in May, a month after witnesses say they were approached to provide their phones.
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WhatsApp, for instance, markets itself as a way to securely communicate with people overseas.
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Dust dubs itself a "safer place to text," and pushes its platform as a way to keep messages secretive as well as giving their users the ability to erase messages off of other people's phones, according to their website.
Dust was also the app reportedly used between longtime Trump personal attorney Michael Cohen and Felix Sater, a real estate developer who has claimed to have ties to Russian oligarchs, when they tried to complete a deal for Trump Tower Moscow. The plan ultimately fell apart.