"The tension between the benefits and challenges of encryption has been an issue for law enforcement and policymakers since the 1990s, and was reinvigorated in 2014 when companies like Apple and Google implemented automatic enhanced encryption on mobile devices and certain communications systems. Companies using such strong encryption do not maintain 'back door' keys and, therefore, now cannot easily unlock, or decrypt, the devices--not even when presented with a valid legal order. Law enforcement concerns about the lack of back door keys were highlighted by the November and December 2015 terrorist attacks in Paris, France, and San Bernardino, CA. Questions arose as to whether the attackers used strong encryption and, more importantly, if they did, whether and how this might have hindered investigations. Following the December 2, 2015, terrorist attack in San Bernardino, CA,, U.S. investigators recovered a cell phone reportedly used by one of the shooters. Federal Bureau of Investigation (FBI) Director James B. Comey testified before Congress two months later, indicating that the Bureau was still unable to access the information on that device. On February 16, 2016, the U.S. District Court for the Central District of California ordered Apple to provide 'reasonable technical assistance to assist law enforcement agents in obtaining access to the data' on the cell phone."
CRS Report for Congress, R44396
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html