The leaders of U.S. and European law enforcement and intelligence agencies have been sending a serious warning: commercially available communication devices equipped with end-to-end encryption software make it impossible for security services to track terrorists plotting an attack – or monitor the terrorists’ communication while the attack is under way.

It was revealed that the terrorists, who perpetrated attacks in Paris on November 13th used the encrypted WhatsApp and Telegram messengers apps to communicate before the attacks – and with each other during the attacks.

What was said in those encrypted messages, and who sent and received these messages, may never be known, because the companies themselves do not have the key – or back door – to decrypt these messages. Thus, security services could not monitor such messages before an attack in order to prevent it, and cannot read these message after an attack to learn more about the terrorists’ network and support system.

FBI director James Comey last week told Congressmen that one of the suspects in the foiled terror attack in Garland, Texas, in May 2015 had exchanged 109 messages with sources in a “terrorist location” overseas ahead of the attack.

U.S. intelligence and law enforcement agencies, however, have not been able to break into and read those messages because they were exchanged on devices equipped with end-to-end encryption software.