Three members of a work crew died at the scene of an April 2 blast in Anacortes, Wash., and two others died later.
Company officials said the explosion and fire occurred in a naphtha unit of the plant. Workers had finished routine maintenance on a boiler in the unit and were in the process of increasing the heat and pressure to operating levels when the fire erupted.
Tesoro Corp., the operator of the refinery, said it would curtail operations at the refinery until treating capacity can return to service.
The company said it couldn't predict when operations would return to normal. "However, progress on the various investigations of the incident and the completion of unit repairs are both necessary for a restart," Tesoro added in a written statement.
The refinery has been producing unfinished intermediate products since the April 2 fire that cannot be refined until the damaged units return to service or the company explores an "alternate plan."
"Tesoro is committed to our employees, the community and our customers," said Tesoro Chairman Bruce Smith. "We will fulfill our commitments to them during this difficult time."