Communications of the ACM

A hacking group believed to have links to the Chinese government penetrated the Metropolitan Transportation Authority's computer systems in April, exposing vulnerabilities in a vast transportation network that carries millions of people every day, according to an M.T.A. document that outlined the breach.

The hackers did not gain access to systems that control train cars and rider safety was not at risk, transit officials said, adding that the intrusion appeared to have done little, if any, damage.

But a week after the agency learned of the attack, officials raised concerns that hackers could have entered those operational systems or that they could continue to penetrate the agency's computer systems through a back door, the document also shows.

Transit officials say a forensic analysis of the attack has not revealed evidence of either and that hackers did not compromise customers' personal information. The agency reported the attack to law enforcement and other state agencies, but has not disclosed it publicly.