The explosion, which occurred Feb. 18, 2015, rocked Torrance like an earthquake, and smoke rose from the refinery in the heart of town. The blast injured four contractors and spewed a cloud of ash onto the nearby community.
Two years later, U.S. Chemical Safety and Hazard Investigation Board officials say they think they know what went wrong.
"The CSB found that this incident occurred due to gaps in the refinery's process safety management system," said Vanessa Allen Sutherland, chair of the CSB. "Finally, a critical safeguard within the system was degraded to the point of failure."
The report also stated that the explosion nearly resulted in the rupture of a tank storing thousands of pounds of toxic hydroflouric acid. Officials with ExxonMobil, which owned the refinery at the time, disputed that there was ever a threat of a hydroflouric acid release "or risk of harm to the community."
MORE: Torrance City Council votes against phasing out hydroflouric acid at refinery
The CSB does not issue any citations, but the report makes a series of recommendations for improving safety at the facility.
Federal inspectors said workers were running the refinery blind, and ExxonMobil isn't cooperating fully with the investigation.
ExxonMobil said Wednesday it regrets the incident and will work with the board to understand the findings and recommendations
VIDEO: AIR7 HD over scene of explosion at Exxon Mobil refinery
For local first responders, the report leaves important questions unanswered.
"In life, I'm never going to have all the answers, so we do the best we can preparing, again, for the safety of the community here," said Capt. Robert Millea with the Torrance Fire Department. "Sounds like they're still requesting information."
Shortly after the 2015 explosion, ExxonMobil sold the refinery. The refinery is now owned by PBF Energy, which has yet to comment on the CSB report.
The Associated Press and City News Service contributed to this report.