A poorly crafted joint in a plastic Con Edison gas line and an 8-year-old break in an old city sewer line were the likely causes of an explosion that killed eight people in New York City last year, federal investigators said Tuesday.

The weakness of the plastic pipe joint was exposed because the soil that supported it was washed away by groundwater flowing into a gaping hole in the brick sewer line, the National Transportation Safety Board said.

Both Con Ed and the city took issue with the conclusion, each claiming the other was fully responsible for the March 12, 2014, blast. The morning explosion also injured 50 people, left more than 100 families homeless and disrupted train travel by throwing debris onto the Metro-North Railroad tracks above the street.

Con Ed blamed the sewer breach entirely, saying, “Not all of the participants involved in this investigation reached the same conclusion concerning the sequence of infrastructure failures.”

City spokeswoman Amy Spitalnick said blaming the sewer line “appears unsupported by the facts,” noting that the breach was 43 feet from the gas pipe connection.

“The full investigation reveals that a properly fused fusion joint would not have failed,” she said.

The NTSB said the worker who made the saddle joint in the plastic gas line in 2011 — so a new building could get gas from the main — failed to ensure that the two surfaces were clean. That contaminated the joint, which was made by melting the plastic, the agency said.

The defective joint “was the only credible source of natural gas that could have provided a large enough flow rate” to fuel the explosion, the NTSB said. But the joint opened up because the gas line was sagging as a result of the erosion beneath it, the board said.

If the break in the sewer main had been repaired after it was detected in 2006, the explosion might have been prevented, the board said.

It said that when a report of a gas odor was called in to Con Ed on the day of the explosion, a dispatcher notified the city fire department, but when the department called back for an address, the dispatcher said, “Hold up. No, sorry. Hold up one second. Hold on. I will call you back. I will call you right back,” but did not follow up.

The NTSB’s staff analysis said the fire department could have reached the scene 15 minutes before the explosion and perhaps would have begun an evacuation. During a discussion, however, NTSB members cautioned that it’s not clear any lives would have been saved.

Other findings included:

—Several people reported after the explosion that they had smelled gas the day before, but none called it in.

—Con Ed’s public education program did not effectively tell customers what to do when detecting a gas odor.

—If Con Ed had installed appropriate valves on the gas line, the leaking main could have been isolated and turned off more quickly after the explosion.

—The gas line installer’s qualification credentials were not up to date.

The board recommended that Con Ed revise its plastic welding procedures; provide guidelines and training on how to notify the fire department of an emergency; and install more isolation valves.

Con Ed said — and the NTSB agreed — that it has already implemented several remedies.

“We agree with many of the NTSB’s recommendations, and many new gas safety and quality control measures are already underway,” the company said.

The NTSB also recommended that New York City implement new procedures to ensure the integrity of sewer lines and make timely repairs.

(AP)