The decision by Bay Area Rapid Transit officials to cut off cellphone service Thursday evening – to forestall a planned protest – raises a fundamental question: Do Americans have a basic right to digital free speech or to digitally organized assembly?

Because July protests against BART police shootings had turned violent, BART officials took the unusual step to protect public safety, they said. The tactic may have worked: No protests took place Thursday night at BART stations.

Temporarily shutting down cell service and beefing up police patrols were "great tool[s] to utilize for this specific purpose," BART police Lt. Andy Alkire told Bay City News Friday. The protests, planned for sometime between 4 and 8 p.m. in transit stations, would likely have disrupted service for many of the 341,000 daily BART passengers.

This may be the first time a government agency in the United States has ever deliberately disrupted cellphone service to defang planned protests, criminologist Casey Jordan told CNN. “I haven’t been able to find another incident in which this has happened,” she told CNN’s Suzanne Malveaux Friday.

Iran used cell-phone-jamming technology to hobble protests in 2009, and Britain has considered doing so to thwart the social-media-driven riots that dogged London and other locales this week.

The Federal Communications Commission (FCC) forbids jamming cellphones, but BART's move had a different legal context. Because the transit system contracts with five large telecommunications firms to provide underground and station service, BART did not use jamming technology, it simply turned off a service.

Some riders complained to CNN that the decision to turn off cellphone service punished the many for the actions of a few.

First Amendment experts agree that the broadbrush move pushed the bounds of free speech rights.

"Government can legitimately stop speech for public safety purposes, but it has to be the narrowest possible response, it has to be reasonable, and there has to be an imminent threat," says Gene Policinski, executive director of the First Amendment Center at Vanderbilt University, in Nashville. "It can't be done on mere speculation."

Government agencies have been slow to address the burgeoning use of social media to foment protests and recent "flash mob" robberies and attacks in the US.

"I think you can on the one hand argue it was a momentary discomfort for somebody who has other means of communication," says Professor Policinski. "On the other hand, it's a very disquieting development. Here you have a government agency indiscriminately closing down all kinds of speech in order to prevent a perceived possibility of violence."

At the same time, BART may have been within its rights, said Professor Jordan, a criminologist at Western Connecticut State University, in Danbury. “They didn’t try to shut down the protest," she told CNN. "They simply turned off the cell service so it couldn’t become viral. It really is just a cost/benefit analysis of where your freedom of speech begins to threaten the public safety.”

BART officials said they had reliable intelligence that a protest was planned. And, in fact, CNN interviewed one man on a BART platform who said he was there to participate.