"We are deeply concerned about the safety and of our employees and their families," BART's interim general manager, Sherwood Wakeman, said in the statement. "We stand behind them and our customers who were the subject of an earlier attack. We are deeply troubled by these actions."

As of late Wednesday afternoon, the BART police union website from which the data was accessed remained unavailable.

The attacks by Anonymous against BART are similar to numerous other attacks the group has carried out recently. Over the last one year, its victims have included PayPal, Amazon.com, Sony and Booz Allen Hamilton.

In June, Anonymous was labeled a cyberterrorism group by the Arizona Department of Public Safety after members of the group repeatedly attacked Arizona police union.

Most of the attacks have been in retaliation for some perceived wrongdoing on the part of the targeted organization. PayPal for instance was attacked for its perceived opposition to WikiLeaks, while the Arizona law enforcement websites were targeted to protest the state's tough immigration laws.