HOUSTON, April 25 (Reuters) – The seventh person injured in a deadly April 2 fire at Tesoro Corp’s (TSO.N) Anacortes, Washington, refinery has died, the company said in a statement issued on Saturday.

The seven workers were engulfed in a sudden blaze that erupted as they were working on a naphtha hydrotreater heat exchanger, investigators have said.

The Tesoro refinery fire is the deadliest refinery calamity since the Texas City, Texas, BP refinery blast in 2005 that killed 15 workers and injured 180 other people.

Three workers were killed immediately in the Tesoro blaze. The others died at a Seattle hospital while being treated for severe burns.

The cause of the fire remains under investigation by Washington state workplace safety regulators and the U.S. Chemical Safety Board.

Tesoro has said it plans to shut the 120,000 barrel-per-day (bpd) refinery late this month for a temporary, but unspecified period of time to repair the refinery’s hydroprocessing equipment that was shut down following the fire.

Hydroprocessing equipment, like the hydrotreater, removes pollution from motor fuel. Some hydroprocessing also boosts the amount of material that can be made into motor fuel from crude oil. (Reporting by Erwin Seba, editing by Maureen Bavdek)