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Special counsel Robert Mueller detailed multiple contacts among Russian operatives and associates of President Donald Trump in the report made public Thursday. But Mueller repeatedly also lamented what he couldn't learn — because encrypted communications had put key conversations beyond his reach.

"The Office learned that some of the individuals we interviewed or whose conduct we investigated — including some associated with the Trump Campaign — deleted relevant communications or communicated during the relevant period using applications that feature encryption or that do not provide for long-term retention of data or communications records," Mueller wrote.

This reality, an increasingly familiar one to government investigators, is the product of a shift in communication technologies that has accelerated in recent years: The revelations in 2013 by National Security Agency contractor Edward Snowden about government surveillance of global communications led to extensive new investments in encryption technology and their widespread adoption by many people wary of government intrusion.

What experts call "end-to-end encryption" — meaning only the sender's and recipient's devices can decode messages — once was the province of high-level operatives and the technically savvy but has become mainstream. WhatsApp, which encrypts all messages, is the world's most popular messaging app.

Even the popular messaging apps Apple builds into iPhones — iMessage and FaceTime — are encrypted, making them far more secure than ordinary phone texting.

Mueller rarely named specific encryption tools in his report, but such popular ones as Signal and WhatsApp are used widely in Washington, D.C., and by government officials — spies, diplomats, investigators and others — around the world.

The use of encryption technology appeared to hinder Mueller in his efforts to uncover dealings between Paul Manafort, who was Trump's campaign chairman, and Konstantin Kilimnik, a Russian whom Manafort worked with and U.S. officials believe has connections to Russian intelligence.

The report said, "The investigation did not uncover evidence of Manafort's passing along information about Ukrainian peace plans to the candidate or anyone else in the Campaign or the Administration. The Office was not, however, able to gain access to all of Manafort's electronic communications (in some instances, messages were sent using encryption applications)."

Mueller concluded that encryption technology also was used to obscure the efforts of the GRU, a Russian military intelligence agency, in seeking to disseminate e-mails it had stolen from Democratic Party operatives by sharing the information with anti-secrecy group WikiLeaks.

"GRU officers used both the DCLeaks and Guccifer 2.0 personas to communicate with WikiLeaks through Twitter private messaging and through encrypted channels, including possibly through WikiLeaks's private communication system," Mueller wrote.