An anonymous reader writes: Gut instinct suggests that the network must've been overloaded with people trying to find loved ones. At first, the Associated Press said it was a concerted effort to prevent any remote detonators from being used, citing a law enforcement official. After some disputed that report, the AP reverse its report, citing officials from Verizon and Sprint who said they'd never had a request to shut down the network, and who blamed slowdowns on heavy load.
(Motherboard's Derek Mead was able to send text messages to both his sister and her boyfriend, who were very near the finish line, shortly after the bombing, which suggests that networks were never totally shut down. Still, shutting down cell phone networks to prevent remote detonation wouldn't be without precedent: It is a common tactic in Pakistan, where bombings happen with regularity.)