NTSB begins investigation of East Harlem explosion.
A National Transportation Safety Board official says there are no obvious ruptures or punctures in a gas main and distribution pipe at the explosion site.
Robert Sumwalt said Thursday that further examination is required.
He noted that rescue efforts are still going on at the site.
Sumwalt said investigators are operating under the assumption that a natural gas leak led to the explosion. He says it's unclear whether the leak came from inside or outside the buildings.
Sumwalt said there also was a water main break. But it's not known if that led to the explosion or was caused by it.