Hackers have launched another attack on a transport agency that cut off mobile phone services at San Francisco stations last week to prevent protests.
Hacking group Anonymous announced on Twitter that the private data of 102 Bay Area Rapid Transit (Bart) police officers had been leaked.
The group claimed responsibility on Monday for disabling Bart's marketing website, myBART.org.
Bart police have been criticised for shooting dead a homeless man in July.
Officers said the man lunged at them with a knife, but the killing has caused public anger.
Violation of free speech?
Bart attempted to stop customer protests at some of its train stations on 11 August by disconnecting its mobile transmitters and halting phone services for its passengers.
On Wednesday, Anonymous announced through its Twitter account that the Bart police union website had been breached. It was not immediately clear if Anonymous did it.
The hacker group also supplied a link in the tweet where Bart officers' home addresses, emails and passwords could be found.
The union's website was disabled later on Wednesday.
Bart deputy police chief Daniel Hartwig said he had been made aware of the breach.
Bart interim general manager Sherwood Wakeman said in a prepared statement: "We are deeply concerned about the safety and security of our employees and their families."
Anonymous has called for the Bart police force to disband over the 3 July shooting of the homeless man, and another fatal police shooting in 2009.
Shortly after the police data was leaked, Anonymous posted a message on Tumblr.com saying: "Push the wrong buttons and we will exploit what needs to be released to the public."
The Federal Communications Commission has said it is investigating whether Bart had the right to stop mobile phone services for its passengers.
Bart said on Monday that the company "accommodates expressive activities that are constitutionally protected by the first amendment to the United States constitution".
But some legal commentators have said Bart's move to block mobile phone use was a violation of free speech under the first amendment of the US constitution.
Its actions have been compared to Arab governments' attempts this year to shut internet access in response to mass demonstrations.
The American Civil Liberties Union of Northern California said: "Are we really willing to tolerate the same silencing of protest here in the United States?"
Last week, the UK government said it was exploring whether to turn off social networks or stop people texting during social unrest, following the recent riots there.