The leaking gas main was adjacent to 1646 Park Avenue, one of the buildings destroyed in the March 12 blast, the National Transportation Safety Board said.
According to the NTSB, the leak was detected during a pressure test on the 8-inch main beneath Park Avenue. A tracer gas escaped under normal operating pressure, investigators said.
Authorities have said the blast erupted about 15 minutes after someone from a neighboring building reported smelling gas. And the NTSB said Friday that that underground tests conducted in the hours after the explosion registered high concentrations of natural gas.
However, board spokesman Eric Weiss said investigators were far from deciding what caused the explosion.
"We don't determine the probable cause until the very, very end," he said.
The NTSB said it will excavate the area near the leak to expose the gas main, parts of which are made of cast iron and date back to 1887.
A camera probe will be run inside both the gas main and a damaged water main on Park Avenue and the resulting video will be examined, the board said.
The NTSB has said it was unclear if the broken water main somehow contributed to the explosion or was caused by it. The water main dates back to 1897.
The board also said segments of the gas service lines into the two buildings have been recovered from the basements and will be shipped to the NTSB lab in Washington for tests. A cracked segment of the water main will also be sent to the lab.
The NTSB said tests on service lines into adjacent buildings have shown "no significant findings" so far.
Officials said Monday that the victims died of blunt trauma, smoke inhalation or burns.
They were identified as Griselde Camacho, 45, a Hunter College security officer; Carmen Tanco, 67, a dental hygienist; Andreas Panagopoulos, 43, a musician from Greece; Rosaura Hernandez, 22, a restaurant cook from Mexico; her mother, Rosaura Barrios, 43; George Amadeo, 44, a handyman; Jordy Salas, 22, a restaurant worker; and Mayumi Nakamura, 34, from Japan.