Criminals use cell phones. And because police agencies know this, they tend to be a little more cautious about said phones than regular users. Cautious enough to, say, buy a special fully-encrypted phone that purports to be 100% untraceable, and use the completely hack-proof messaging app contained within. Some of those criminals came to regret it as they discovered their super-secret phones and messaging service were, in fact, provided by the United States Federal Bureau of Investigation and other police forces around the world.

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