Updated 10:31 PM ET
The website of the U.S. Department of Justice was shut down Thursday in what the FBI called a denial of service attack following a piracy indictment involving the file-sharing site Megaupload.
The public websites of Universal Music and the Motion Picture Association of America were also shuttered. On Twitter, the loosely-organized hacking group Anonymous claimed responsibility for the hacks.
By late Thursday evening, the homepage of the Justice Department's website was back in operation.
Internet relay chats supported by the Anonymous collective show participants discussing the DOJ site being down and talking about other U.S. government sites to target, including the Recording Industry of America.
In addition to the Megaupload indictment, the activists are angry about two proposed antipiracy bills backed by the music and movie industries,, that critics say would give authorities broad power to shut down websites for the mere accusation that they had pirated content on them.
Numerous websites including Wikipedia and Reddit "blacked out" on Wednesday, deliberately hiding their normal content and posting messages protesting the antipiracy bills. Many U.S. entertainment and media companies, including CBS Corporation, back the legislation.Full coverage of SOPA, PIPA at Tech Talk
"Seems like some friendly ships are launching torpedos justice.gov as we speak. The site seems down to us! (via @AnonOpsSweden)," Twitter accounts associated with the Anonymous online activist group posted Thursday following the apparent hacks.
Another tweet by the AnonOps account said: "Tango down! universalmusic.com & justice.gov// #Megaupload."
Shortly before the outage, seven people were named in an indictment and four were taken into custody on online piracy charges, including Kim Dotcom, aka Kim Schmitz, the founder of Megaupload, the internet file-posting service that claimed 50 million daily users.
The site allows users to post digital files - text, movies, music, computer programs, and more - and provide a link for others to download them for free. It charged users for premium accounts that permitted faster and more frequent downloads.
A DOJ spokeswoman said she was trying to get comment on the matter and Universal Music had no comment.
CNET's Elinor Mills contributed to this report