Hacktivist group Anonymous has hit San Francisco's rail transport authority for a second time.
Anonymous claims to have hacked Bay Area Rapid Transit amid continuing protests against the killing of a knife-wielding homeless man by transit police.
The hacker group announced on Twitter that personal data of 102 Bay Area Rapid Transit (Bart) police officers had been leaked, according to the BBC.
The group claimed the Bart police union website had been breached, providing a link to the captured personal details. But it was not immediately clear if Anonymous was responsible for breaking into the union website, which was later taken offline, the report said.
Anonymous later posted a message on Tumblr.com saying: "Push the wrong buttons and we will exploit what needs to be released to the public."
The group has called for the Bart police to be disbanded over the killing of the homeless man and the shooting of another man in 2009.
"The Bart Police Officers Association has been caught with its pants down," says Graham Cluley, senior technology consultant at security firm Sophos.
"It seems likely that a vulnerability on their site will have let the hackers access the police officer's database. Clearly the information had not been properly secured," he wrote in a blog post.
"In the current climate of high profile hacks that's not excusable. Other forces would be wise to look at their own sites and make sure that they are not similarly vulnerable to attack," Graham Cluley said.
Anonymous previously defaced and disabled the Bart's marketing website MyBart.org and released user details. The hacktivist group said it launched the cyber attack in retaliation to Bart's decision to disable mobile phone transmitters at some stations after learning demonstrators planned to use mobile phones to co-ordinate protests.
The US Federal Communications Commission (FTC) has announced it is conducting an investigation into whether Bart had the right to stop mobile phone services for its passengers.
Bart's decision to block mobile communications has met with similar criticism from civil liberties groups, as UK Prime Minister David Cameron's suggestion in Parliament that access to social networks could be blocked in times of civil unrest.
David Cameron's comments followed reports that riots and looting in the UK had been co-ordinated using social media and encrypted text messaging services such as Blackberry Messenger.