To back up the claim, a list of over one million iPhone and iPad UDIDs (Unique Device Identifiers) has been released.
The hackers claim that a Java vulnerability was used to extract files from a notebook "used by Supervisor Special Agent Christopher K. Stangl from FBI Regional Cyber Action Team and New York FBI Office" in March 2012.
One file "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."
What else has been done with the data during the last six months is unknown.
The name of the file in question was NCFTA_iOS_devices_intel.csv.
NCFTA is the National Cyber-Forensics and Training Alliance, which describes itself as "a conduit between private industry and law enforcement with a core mission to identify, mitigate and neutralize cyber crime."
According to an FBI spokesperson, "At this time there is no evidence indicating that an FBI laptop was compromised or that the FBI either sought or obtained this data."