The self-dubbed Crackas with Attitude also posted online a purported list of 3,500 names, phone numbers and email addresses of U.S. authorities and military personnel.
This story has been updated with a response from the FBI.
Teenagers bent on embarrassing U.S. government officials claim to have broken into several federal systems, as well as obtained private contact details for thousands of law enforcement and military employees.
Last month, the same group took responsibility for hacking the home AOL account of CIA Director John Brennan.
According to Motherboard, now the self-dubbed Crackas with Attitude say they have breached the FBI's Internet Crime Complaint Center -- a portal for reports about online scams; JABS -- a conduit for information on booked criminal suspects; and the bureau's Virtual Command Center -- an information-sharing hub for crisis management.
The hacker activists also posted online a purported list of 3,500 names, phone numbers and email addresses of U.S. authorities and military personnel.
On Friday evening, an FBI spokeswoman told Nextgov in an email that the bureau has no comment on "specific claims of hacktivism" but went on to state that the government will find the people responsible for such crimes.
"Those who engage in such activities are breaking the law," she said. "The FBI takes these matters very seriously. We will work with our public and private sector partners to identify and hold accountable those who engage in illegal activities in cyberspace."
The Department of Homeland Security had no comment. The Pentagon referred questions to the FBI.
One group member, who calls himself "Cracka," told Motherboard the gang busted into someone’s account, and got access to several “tools feds use” including the three databases. Cracka said the victim is someone “high in the [government].”
He said the group, which describes itself as a bunch of stoners, did not copy all of the information at its fingertips.
“We let the [government] off by a lot, this could be so damaging it could affect the whole of USA by ALOT [sic],” Cracka said in an online chat.
In an earlier tweet, Cracka boasted having “34,000 lines of emails, names, position and phone numbers of gov associates, including military,” suggesting the gang might have more data to leak. “Just to clear this up, CWA did, indeed, have access to everybody in USA's private information, now imagine if we was [sic] Russia or China,” he said in another tweet.
Motherboard reports the publication was able to confirm the accuracy of at least five numbers in the list.
Earlier in the week, the hacktivists asserted they had entered a Comcast email account owned by the family of FBI Deputy Director Mark Giuliano. Cracka, at the time, told Motherboard he spoke with the bureau official on his cell phone number, which was listed in the account's contacts. When Motherboard called the number, it went over to a voicemail greeting belonging to a Mark Giuliano.