A tank explosion and fire at the Caribbean Petroleum Refining facility near San Juan, Puerto Rico, in October was likely caused by an accidental overfilling of a tank with gasoline, the United States Chemical Safety Board reported. The agency is conducting a full investigation of the Oct. 23 incident.

According to the news release, at the time of the incident a tank was being filled with gasoline from a ship docked at San Juan harbor. Investigators have determined that a likely scenario leading to the release was that the tank was accidentally overfilled, which caused an undetected gasoline spill. As the material spilled, it vaporized and spread across the facility. CSB investigators estimate that the vapor cloud spread to a 2,000 foot diameter until it reached an ignition source in the northwest section of the facility.

The CSB found that on the evening of the incident, the liquid level in the tank could not be determined because the facility’s computerized level monitoring system was not fully operational. In order to monitor the level in the tank, operators used a mechanical gauge on the tank’s exterior wall. Therefore employees located in the facility’s control room were unaware of the emergency.

“The filing of a tank without a functioning monitoring system is the type of activity the CSB will be examining very closely,” said Investigator-in-Charge Jeffrey Wanko, P.E. CSP.

“The CSB’s investigation will examine operations particular to Caribbean Petroleum, but will also look at the regulations and best practices surrounding the industry as a whole in an effort to improve safety practices at similar facilities,” he continued.

The large vapor cloud ignited at the Caribbean Petroleum facility at 12:23 AM ET on Oct. 23, damaging homes and businesses located more than a mile from the facility. Over the past few weeks, the five-person investigative team from CSB has conducted numerous interviews, requested hundreds of pages of documents and cataloged key pieces of evidence.

Caribbean Petroleum is a significant petroleum products supplier for Puerto Rico. The facility includes a tank farm and a refinery that was shutdown in 2000. Prior to Oct. 23, the tank farm stored gasoline, diesel fuel, jet fuel and fuel oil in approximately 30 operational aboveground storage tanks.