The spill happened at a Gas-N-Go location at Coebourn Boulevard and Edgmont Avenue in Brookhaven, Pennsylvania.
Sources tell Action News that while the investigation is just beginning, the probe is currently centering on the fuel truck that made a delivery there late Friday night.
Officials do know an alarm for high tank levels went off, but the fire department was not made aware until Saturday, the next day, after they got reports of a strong odor, officials said.
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"It's hard to believe that whoever did this, or is responsible, didn't know and didn't report it in time. The consequences from this not being reported by someone responsible is we are eight hours behind in the cleanup and remediation," said Tim Boyce, the county's director of emergency services.
While Deputy District Attorney Douglas Rhoads confirmed this is a criminal investigation, he says there are still more questions than answers.
"Who did what? What happened? What should have happened? These are questions that we seek to answer in a criminal investigation," he said.
Meanwhile, the cleanup continued on Monday as crews did everything possible to keep the environmental damage contained.
"Dead fish, dead fowl, dead foxes laying there," Boyce said. "It's really heartbreaking to this neighborhood."
It's expected to take several months for crews to clean up the fuel spill, according to Brookhaven council president Terry Heller.
They believe it'll take years for the environment to recover as crews continue to find dozens of dead fish in local streams and retention ponds, Heller said.
Officials said anywhere between 1,000 to 4,500 gallons of fuel spilled into area waterways.
So far, officials said the spill is contained to local streams and retention ponds and hasn't affected the nearby Chester Creek or the Delaware River, which is roughly 3.5 miles away.
Lee Transport LLC General Manager Fred Terpolilli said in a statement to 6abc:
"I have a 3rd party group doing an investigation on behalf of the Lee Transport Systems LLC. It would be premature of me to make any statements until the root cause of the spill is determined. I do not want to undermine the investigators by giving inconclusive information."