Saboteurs stole passwords and sensitive information on tipsters while hacking into the websites of several law enforcement agencies worldwide in attacks attributed to the collective known as Anonymous.
Breaches were reported this week in Boston, Syracuse, N.Y., Salt Lake City and Greece.
Hackers gained access to the Salt Lake City Police Department website that gathers citizen complaints about drug and other crimes, including phone numbers, addresses and other personal data of informants, police said.
The website remained down Friday as police worked to make it more secure.
Anonymous is a collection of Internet enthusiasts, pranksters and activists whose targets have included financial institutions such as Visa and MasterCard, the Church of Scientology and law enforcement agencies.
Following a spate of arrests across the world, the group and its various offshoots have focused their attention on law enforcement agencies in general and the FBI in particular.
The group also claimed responsibility for hacking the website of a Virginia law firm that represented a U.S. Marine involved in the deaths of civilians in Iraq in 2005.
Anonymous also published a recording on the Internet Friday of a phone call between the FBI and Scotland Yard, gloating in a Twitter message that "the FBI might be curious how we're able to continuously read their internal comms for some time now."
FBI spokeswoman Jenny Shearer said in an email to The Associated Press the agency was aware of the incidents, and an investigation was ongoing.
In Greece, the Justice Ministry took down its site Friday after a video by activists claiming to be Greek and Cypriot members of Anonymous was displayed for at least two hours.
In Boston, a message posted on the police website before it was taken down Friday said, "Anonymous hacks Boston Police website in retaliation for police brutality at OWS," an apparent reference to the Occupy Wall Street movement. The message also promised "there is plenty more mayhem to deliver."
A police spokesman would not confirm Anonymous was responsible.
Another message on the department's website said a hack several months ago unearthed hundreds of passwords that were released in retaliation for what was called brutality against Occupy Boston.
In October, Boston police acknowledged that various websites used by members of the police department _ including the website belonging to the police patrolmen's association _ had been hacked and possibly compromised. The department said it asked all police personnel to change their passwords on its network.
The Occupy movement in Boston set up camp in the city's financial district for two months this fall. The first hack came about 10 days after Boston police arrested 141 Occupy demonstrators on Oct. 11.
Police dismantled the camp Dec. 10, citing public health and safety concerns.
"So you get your kicks beating protesters? That's OK; we get kicks defacing ... your websites _ again," the message on the department's website said Friday.
Boston police called it unfortunate that the hacking has interrupted the department's ability to inform the community about important safety matters.
Salt Lake City authorities continued their investigation and said criminal charges were being considered.
Police said Anonymous had taken credit for the attack through local media but hadn't contacted the department directly.
The hackers claim to have targeted the site in opposition to an anti-graffiti paraphernalia bill that eventually failed in the state Senate. The bill would have made it illegal to possess any instrument, tool or device with the intent of vandalizing an area with graffiti.
Salt Lake City police Detective Josh Ashdown downplayed any danger to citizens.
He said the department's website is used by residents to report crimes or suspicious activity, and that some submit the tips anonymously while others include personal information.
Ashdown said investigators believe the group is bluffing about the extent of the information it got from the website, and he noted authorities didn't think any of the details would be widely distributed.
He said police don't have any reason to believe that citizens who reported crimes on the website are going to be targeted specifically.
"Our main concern is for the public not to lose confidence in the department," Ashdown said.
In New York, Syracuse police said the department website had also been hacked in an attack attributed to Anonymous.
Sgt. Tom Connellan said names and passwords of people authorized to alter the site were stolen earlier this week and posted on Twitter.
No private information about officers or citizens was accessed, he said, though the site remained down Friday while the FBI and state police continued to investigate. In an online post attributed to Anonymous, the group claims to have targeted the Syracuse site for failing to aggressively pursue child abuse allegations against a former assistant basketball coach.
Another incident struck the website of the Alexandria, Va., law firm of Puckett & Faraj, which represented a U.S. Marine convicted of negligent dereliction of duty in a 2005 attack in Haditha, Iraq, that resulted in the deaths of 24 unarmed civilians.
Attorney Neal Puckett did not immediately return a telephone message and email seeking comment Friday, and the firm's website remained offline.
Lavoie reported from Boston. Associated Press writer Raphael Satter in London contributed to this report, Pete Yost and Lolita Baldor from Washington, and Julie Watson from San Diego.