The hacking collective AntiSec has released more than a million iPhone ID numbers that it said it took from an FBI computer.
The group claims to have more than 12m Apple Unique Device Identifiers (UDIDs).
It said the IDs were listed alongside user names, device names, phone numbers, addresses and notification tokens.
The group said it suspects the FBI had the information to track iPhone users. It did not release all of the information it claims to have, but said there was enough for users to search for their own devices.
In a long statement published online, the group said "we decided we'd help out Internet security by auditing FBI first."
During the second week of March 2012, a Dell Vostro notebook, used by Supervisor Special Agent Christopher K. Stangl from FBI Regional Cyber Action Team and New York FBI Office Evidence Response Team was breached using the AtomicReferenceArray vulnerability on Java.
"During the shell session some files were downloaded from his Desktop folder one of them with the name of "NCFTA_iOS_devices_intel.csv" turned to be a list of 12,367,232 Apple iOS devices including Unique Device Identifiers (UDID), user names, name of device, type of device, Apple Push Notification Service tokens, zipcodes, cellphone numbers, addresses, etc. the personal details fields referring to people appears many times empty leaving the whole list incompleted on many parts. no other file on the same folder makes mention about this list or its purpose."
The group also said it was releasing the file to make sure that people "pay attention" to the alleged surveillance by FBI officers.
The group added that it won't give interviews to journalists until a writer for the American news website Gawker publishes a photo of its writer Adrien Chen, who has criticised Anonymous and other groups in the past, in a ballet tutu.
Meanwhile, the group concluded by offering its unique brand of advice to the rebels fighting Bashar Assad in Syria:
"To Syrian rebels: If Assad wins he will exterminate all of you till the very last one, so better go and kill the motherf****r and his bunch of suckers for once. Support to Pussy Riot: Hang in there, babes! Resistance forever."