BART officials said they switched off the transit system's underground cell phone network for three hours during Thursday afternoon rush hour to prevent protesters from coordinating plans to stop trains. The protesters, which had briefly shut down some stations in July, made online statements saying they planned to use mobile devices to coordinate a protest of the fatal July 3 shooting of a knife-wielding man, Charles Blair Hill, by BART police.

Even though BART owns and operates the wireless network strung through its subways and says it was only interested in keeping its platforms safe, it’s coming under heavy fire from civil rights groups who question the constitutionality of the action.

BART’s deputy police chief said he decided to switch off the service out of concern that protesters on station platforms could clash with commuters, create a panic and put people in the way of speeding trains or the high-voltage third rails.