PHILADELPHIA - A week after the largest oil refinery on the East Coast went up in flames after a series of explosions, investigators are learning more about the cause.
The U.S. Chemical Safety Board now says there was a fundamental failure in the safety systems that were in place at the refinery.
Investigators say there was a release of vapor in the alkylation unit and the vapor reached an ignition source, which sparked the fire and then several explosions.
The EPA and OSHA are continuing to conduct their own investigations.
MORE: South Philadelphia oil refinery to close after fire, explosion| Thousands impacted as oil refinery and hospital announce closures | Gas prices could rise after fire-stricken Philadelphia refinery closes
On Monday, Philadelphia Energy Solutions sent notice to state labor officials that it will shut the plant down and lay off about 1,020 workers over the next several weeks.
The fire significantly damaged equipment and systems at a complex that was already struggling financially, the company said.