Rescuers search site of New York building collapse; seven dead

An earthmover clears debris at the site of yesterdays explosion and building collapse in East Harlem in Upper New York March 13, 2014. REUTERS/Marcus Santos/Pool

NEW YORK (Reuters) - Search crews picked through still-smoldering rubble on Thursday, looking for survivors of a gas explosion that caused the collapse a day earlier of two New York City apartment buildings, killing seven people and injuring about 40.

Facing thick smoke and braving bitter cold, dozens of firefighters, police officers and federal investigators were at the scene in Upper Manhattan to determine what caused the explosion at about 9:30 a.m. (13:30 GMT) on Wednesday.