The National Transportation Safety Board announced Tuesday that federal investigators found a gas leak in a natural-gas main near one of the two East Harlem buildings that collapsed last week. The collapse, which came after an explosion, killed eight people. The gas main is made of 127-year-old cast iron as well as new plastic. The news about the gas main came after soil samples near the explosion pointed to a possible gas leak. However, experts stressed that the news about the main does not prove that an underground gas pipe leak caused the disaster.