Story highlights The investigation is ongoing and far from complete

Tech companies have resisted making any changes to their encryption standards to allow compliance with court-ordered warrants

Washington (CNN) Investigators of the Paris attacks have found evidence they believe shows some of the terrorists used encrypted apps to hide plotting for the attacks, officials briefed on the investigation tell CNN.

This is the first time investigators have made that assertion.

Among the apps officials found used by the terrorists were WhatsApp and Telegram, both of which boast of end-to-end encryption that protects the privacy of their users and are difficult to decrypt.

Previously officials have said they found encrypted apps on cell phones recovered from the crime scenes. But they stopped short of saying they believed the apps were used in plotting the attacks.

The officials didn't say what specific evidence shows the apps were used for preparing the attacks. The apps were used in communications among the terrorists for a period before the attacks, the officials say. What was said in encrypted messages may never be known, the officials said.

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