The NTSB, which investigates pipeline accidents, said in a written update Tuesday that the leak was found in the main adjacent to 1646 Park Ave., one of the collapsed buildings.
Officials had previously said that they suspected a gas leak was the cause, but had not found the source.
The buildings erupted about 15 minutes after someone from a neighboring building reported smelling gas, authorities said. Con Edison said it immediately sent workers to check out the report, but they got there too late.
The NTSB said Tuesday that investigators found the leak after the 8-inch cast iron and plastic main failed pressure tests.
The agency said it was taking two pieces of the service line to the two buildings, one 20 feet long and the other 3 feet long, into evidence for further study.
