Oil firm Tesoro, which owns the refinery in Washington state, has launched its own inquiry. The plant was recently fined $85,700 (56,350) for safety violations amid what federal watchdogs call a troubling trend of serious accidents at US refineries.
Three men died at the scene on Friday and two women died later at a Seattle hospital, while two men who were badly burned are in a critical condition. It was the largest fatal refinery accident since a 2005 explosion at a BP plant in Texas killed 15 people and injured another 170.
The blast shook houses and woke people miles away at about 12.30am on Friday, shooting flames as high as the refinery's tower before the blaze was extinguished about 90 minutes later.
"We could tell this was horrific, this was huge," said Jan Taylor of La Conner, Washington, who felt the blast rock her motorhome at a recreational vehicle park across the bay. Six investigators with the US Chemical Safety and Hazard Investigation Board attended the scene, and the Washington Department of Labour and Industries has launched an investigation.
The company said employees were doing maintenance work on a unit that processes highly flammable liquid derived during the refining process.
"It's a volatile process," a Tesoro spokesman said. "We are diligent about being safe."
Last April, inspectors found 150 instances of deficiencies at the refinery, saying Tesoro did not ensure safe work practices and failed to update safety information when changes were made to equipment.