Federal investigators found a leak in a 125-year-old gas main adjacent to one of the two East Harlem buildings that exploded last week, confirming what many suspected was the cause of the blast that killed eight people and injured 58 others. According to a statement from the National Transportation Safety Board, an eight-inch cast-iron and plastic gas main under Park Avenue between 116th and 117th streets failed a test that pumped in tracer gas under normal operating pressure. The NTSB, which had been waiting for rubble to be cleared, sent several pipe segments to its Washington, D.C. lab for further analysis. The source of the spark has not been revealed, and the safety board did not officially confirm that the leak caused the explosion.