SAN FRANCISCO - The agency that oversees the San Francisco area transit system on Thursday authorized police to turn off wireless communications in train stations, but only for extraordinary threats.

The board of directors for Bay Area Rapid Transit approved the new policy Thursday. It limits blackouts to cases in which train passengers, employees or property are threatened or a substantial disruption to train service is possible.

"The full intent and the only intent is to preserve the safety of the patrons, the public and BART employees," said board vice president John McPartland.

The board included two sentences in its policy sent by the Federal Communications Commission acknowledging that cutting cellphone service "poses serious risk to public safety" and should only be done when the benefits outweigh the risks.

Officials crafted the policy after BART was criticized for cutting cellphone and wireless data service in San Francisco subway stations to thwart a planned protest in August. Leaders of the protests had planned to issue demonstration instructions via text messages to protesters once they had massed on a train platform. That protest fizzled after no directions were issued.

Board President Bob Franklin said it's unlikely that BART will again be confronted with similar circumstances because he believes demonstrators won't exclusively rely on electronic communications again

BART became the first known government agency in the nation to block electronic communications as a means to quell social unrest, and the action touched off an outcry among free speech activists and led to cyber-attacks by the hacker collective Anonymous.

Board members were divided over whether the communications disruption was warranted. Board member Lynette Sweet opposed the blackout while others supported the police's decision.