Both Con Edison and New York City were found responsible for the 2014 East Harlem building explosion that killed eight.

The NTSB issued a report, noting that the explosion—caused by a gas leak that leveled two buildings on 116th Street and Park Avenue— "was caused both by a defective pipe joint that allowed gas to leak from a gas main into the building, and an earlier breach in a sewer line that caused the gas main to sag and overstress the defective joint."

Among the findings by the Board was that the surfaces of the service tee and the gas main were not adequately prepared before the tee was fusion-welded to the gas main in 2011 by a contractor to Consolidated Edison Company of New York, Inc., resulting in a defective fusion joint. The Board today also issued a Safety Alert regarding the proper cleaning and surface preparation procedures to ensure strong joints in plastic natural-gas pipelines.

The Board also found that the New York City Department of Environmental Protection (NYC DEP) had learned of a sewer main breach in front of the building on Park Avenue in 2006, but had never repaired it. The supporting soil under the gas line and a water main, in front of the buildings was washed into the sewer through a large hole in the sewer wall.

The sagging gas main led to a crack in the defective service-tee fusion joint, allowing natural gas to escape into the ground and migrate into 1644 Park Avenue.

NTSB Chairman Christopher A. Hart said, "These factors aligned to create the accident, but there were others," and the NY Times reports, "Among them, he said, were the failure of neighborhood residents to report the gas odor they noticed and the failure of Con Edison to notify the Fire Department as soon as somebody alerted the company."

Con Ed is suing NYC over the explosion, claiming that the NYC Department of Transportation knew about "depressions and cave-ins in the roadway at Park Avenue between E. 116th and E. 177th streets multiple times over several years, including the week prior to the incident."

Hart said that the NTSB made a number of recommendation to NYC, Con Ed and the NY State Public Services Commission "to prevent this from happening again. The public’s safety would be greatly advanced by these safety improvements, and the public needs to insist on them... But even if all these recommendations are acted upon, they will not stop every leak. Don’t assume your neighbor reported the gas leak. If you smell gas, first evacuate and move away from the building and then report the leak, either to 911 or to the gas company."

You can read the NTSB's report here (PDF).

Update: The Mayor's office released a statement about the NTSB's findings, "The Board has found that Con Ed's improper installation of a fusion joint at 1642 Park Avenue was a cause of the explosion. Most of the recommendations made today were directed at this probable cause. However, the finding that damage to a sewer may have also played a role appears unsupported by the facts. The sewer was 43 feet north of Con Ed’s faulty gas service connection, and 20 feet deep."

The facts revealed that damage to the sewer and the road depression above it were localized events which had no causal connection to the failure of Con Ed’s gas service connection or the rupture in the City’s water main. The rupture in the City’s water main occurred as a result of the explosion, not due to the damaged sewer.

What the facts presented during the NTSB investigation did reveal was that the explosion was caused by Con Ed’s improper installation of the gas service installation by a Con Ed worker not certified to make that installation.

Put simply, the full investigation reveals that a properly fused fusion joint would not have failed.

Regardless, this tragic incident underscores the need to continue to work closely with Con Ed and all utilities to ensure the highest safety standards.

Over the past year, the City has taken major steps to improve interagency coordination to avoid isolated delays in repairs, to improve responses to potentially hazardous conditions, and to significantly increase and better coordinate investments in water and sewer infrastructure -- so that a tragic incident like this is unlikely to ever happen again.

To that end, we want to remind everyone that if you ever smell gas in your home, apartment or business, leave the location and call 911 immediately.