The April 2, 2010, explosion and fire that fatally injured seven employees at the Tesoro Refinery in Anacortes, Washington was caused by a faulty heat exchanger, according to the U.S. Chemical Safety Board (CSB), which will officially wrap up its investigation into the tragedy at a meeting later this month.

The CSB found that at the time of the incident, a bank of heat exchangers was being brought online in the refinery’s naphtha hydrotreater unit when another heat exchanger in a parallel bank catastrophically failed, spewing highly flammable hydrogen and naphtha which ignited. Seven Tesoro workers who were nearby, assisting with the heat exchanger startup, were fatally burned. 

The accident at Tesoro was the most deadly U.S. refinery incident since the 2005 explosion at BP Texas City that killed 15 workers and injured 180 others.

In addition to a vote on the final investigation report, the upcoming meeting will also produce recommendations intended to prevent similar incidents from occurring at other refineries.