Firefighters and emergency personnel on Thursday dug deeply into the wreckage of two buildings in East Harlem that were leveled in a powerful explosion, looking for survivors and possible evidence of what caused the blast.

But as they cut into a mountain of twisted metal, brick and wood, they found only more victims.

The death toll rose to eight, as four more bodies were found overnight and one later on Thursday. One of the victims was so badly burned that the person could not be identified.

The search for the cause of the gas leak — which a preliminary investigation indicates touched off the collapse of the buildings on Park Avenue near 116th Street on Wednesday morning — was equally elusive, even as new details emerged about the explosion.

Two agents for the Federal Bureau of Investigation saw the blast blow out the front of the piano store located at 1646 Park Avenue, according to two city officials briefed on their statements.