https://www.vice.com/en_us/article/m7vqkv/how-fbi-gets-phone-data-att-tmobile-verizon

A newly obtained document written by the FBI lays out in unusually granular detail how it and other law enforcement agencies can obtain location information of phones from telecommunication companies.

Much of the information reiterates what we already knew about law enforcement access to telecommunications data—how officials can request location data from a telecom with a warrant or use court orders to obtain other information on a phone user, for example. But the document does provide insights on what exactly each carrier collects, a more recent run-down of how long each telecom retains certain types of data for, and images of the tool the FBI makes available to law enforcement agencies across the country to analyze cell phone tower data.