An explosion caused part of a residential building to collapse in the Bronx, according to the New York City Fire Department. FDNY said units responded just after 8 a.m. on Wednesday to 207 Alexander Avenue, where a structure connected to the building had collapsed. Fire Commissioner Robert Tucker said investigators believe the explosion occurred in a ventilation shaft linked to the boiler system. The building is a New York City Housing Authority property. The cause of the explosion remains under investigation by fire marshals, police, and other city agencies.