In the first known case of its kind, U.S. drug agents supplied unwitting cocaine-trafficking suspects in California with smartphones they thought were encrypted but had been rigged to allow eavesdropping, Human Rights Watch reported Friday.

The advocacy group said it feared the technique could be abused to violate the privacy of non-criminals.

"I think there are real debates to be had as to whether that is lawful or should be lawful," said HRW researcher Sarah St. Vincent. "They could use this on peaceful protesters, (though) there's no evidence of that."

Human Rights Watch called on the Drug...