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The blast, which killed seven employees and laid waste to an entire borough block on March 24, was caused by natural gas "leaking from a DuPont Aldyl A service tee that was installed in 1982," NTSB said.
"UGI exposed and retired the service line that was connected to this service tee in 2021 when they relocated the natural gas meter from the basement to the exterior of Building 2," investigators said.
"After the service line was retired, the 1982 service tee remained connected to the natural gas system, pressurized at full system pressure."
A new service tee was installed during the 2021 meter relocation, officials said. That tee was also found to have a "small leak," they added.
NTSB said the 1982 pump's material, Aldyl A, was known by federal regulators to have a "poor performance history" and be prone to cracking since 2007.
The faulty 1982 tee was also "less than two feet from subsurface infrastructures" that ran between Palmer buildings, including "a steam line, a condensate line, and several heated chocolate pipelines," NTSB said.
The steam line was also found to be corroded and cracked when investigators studied it, according to the report.
In their preliminary findings released in May, NTSB investigators said natural gas was the likely culprit.
Around 35 office workers and 70 production employees were in the two RM Palmer buildings on South 2nd Avenue at the time of the explosion around 5 p.m. on March 24, investigators wrote. Workers in both buildings reported "an odor of natural gas" and "the smell of rotten eggs" in the moments leading up to the blast.
In RM Palmer building two, which was completely destroyed by the explosion, workers were sanitizing equipment in the final moments, NTSB found.
Federal regulators said that two UGI pipelines carried natural gas to the RM Palmer site — a 4-inch steel main along South 2nd Avenue in front of building two, and a 1.25-inch plastic main along Cherry Street between the RM Palmer buildings.
However, UGI told NTSB that they were not performing work in the area at the time, and reported "no pressure spike in gas usage before the explosion."
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