The San Franciso subway’s chief spokesman was the brainchild behind Thursday “gut wrenching” decision to shutter mobile-internet and phone service to quell a planned protest, the spokesman, Linton Johnson, said in a telephone interview Tuesday.

His idea, which was approved by the agency’s lawyers and police department, sparked a national debate over whether there was a First Amendment right to mobile phones, and whether the Bay Area Rapid Transit had gone too far in mirroring tactics taken by regimes in the Middle East to stifle dissent.

It was the first time a government agency in the United States had blocked telecommunications service in a bid to hamper a protest.

“It came to me in the middle of the morning,” Johnson said, referring to the idea hours before BART authorities unplugged underground antennas at its four downtown stations during the rush hour commute. “I sent it to the police department and they said, ‘They liked it.’ They started vetting it.”

He said the agency received intelligence that a mob would be using Twitter and mobile phones to organize a protest on the Civic Center subway platform to demonstrate against BART officers killing a knife-wielding man the month before. Johnson said he wanted to prevent another protest like the one that happened on July 11, a week after the killing, in which patrons were seen climbing on trains, he said.

“This was the appropriate tool to ensure our customers’ safety and ensure their First Amendment right to the best we could under these difficult times,” said Johnson, a former San Jose local television reporter.

“It was a gut wrenching decision we had to make,” he said.

A July 11 protester was photographed being pulled from the top of a train during rush hour. He might have been electrocuted or his foot might have broken a train window and injured passengers, Johnson said.

“What if our staff had not managed to grab him on time?” he said.

A protest Monday on the same platform, called by the hacking group Anonymous, ended peacefully and communications service was not shuttered. Johnson said they had no advanced intelligence of what he called “criminal” activity, so telecommunications service remained operational.

“We were told basically, generically, there would be some protests,” Johnson said, noting that subway officials had no advanced warning Monday of any criminal activity.

Last Thursday, when mobile communications were blocked, he said, “They were saying they were going to disrupt train service. Our intelligence found a lot of information of how they were going to cary it out.”