Homeland security officials said the bulletin mentions possible threats to aviation, mass transit and infrastructure in Boston.

One official said suspicious activity has been observed around bridges in the area, and an anonymous phone call threatened that a piece of critical infrastructure would be hit, but the official said there has been no corroborating intelligence.

Because of the security concerns and Logan International Airport's proximity to Boston's port and city center, the official said that more federal air marshals were being deployed on flights in and out of Boston and that passengers flying into the city were under special scrutiny.