A "defective fusion joint" Con Ed installed in 2011 allowed natural gas to leak into the building and ignite, the NTSB found. The surface where the joint was fused to the gas main was not properly cleaned, resulting in contaminations and eventual fractures, the agency found.
The NTSB also placed blame on the New York City Department of Environmental Protection, which it said failed to address a breach in a nearby sewer main that had been damaged for nearly eight years.
The March 12, 2014, explosion destroyed two buildings on Park Avenue, in Manhattan's East Harlem neighborhood. In addition to the dead, 50 people were injured and more than 100 families lost their homes.
In a statement released Tuesday, Con Ed said that in response to the explosion it had drastically increased the number of safety checks it conducts around the city. Gas mains are now serviced once a month rather than once a year, although annual checkups complied with state and federal guidelines, the company said.
Although Con Ed said it agreed largely with the NTSB's findings, it did not completely accept the agency's assessment of blame.
"To be clear, not all of the participants involved in this investigation reached the same conclusion concerning the sequence of infrastructure failures that led to the explosion," the statement read. "We all agree, however, on the importance of doing everything in our control to prevent a tragedy like this from happening again."
The NTSB called on Con Ed to revise its practices for cleaning devices like those found to be faulty at the site of the explosion.
On the day of the deadly blast, someone called Con Ed to report a gas leak, saying they smelled gas outside a building next to one of the two that later collapsed. Utility workers were dispatched to the scene minutes after that 9:13 a.m. call, but did not arrive in time to prevent the explosion.
Within 20 minutes, the blast had reduced both buildings to cinder and ash.
"The explosion occurred before that team could arrive," New York Mayor Bill de Blasio said at the time. "This is a tragedy of the worst kind, because there was no indication in time to save people."
In March of this year, a little more than a year after the East Harlem blast, another gas explosion rocked the city's East Village. Three buildings collapsed, and 19 people were injured.
Con Ed President Craig Ivey told reporters that company officials had been at the scene an hour before that explosion, where they determined a new gas meter that was being installed at the building was not ready to be used.