NBC might be reporting that, but the police commissioner is treating it as an explosion. Getting out of Boston is not so easy, as every Commuter train heading south goes by Back Bay, where the explosion happened, and the subway is closed at Park. Also, internet and cell phone service is very spotty.
The entire cell network of the city area has been shut down by officials to prevent cellular remote detonation. Given reports of two other devices found that may suggest that they were cell phone triggered.