ANACORTES, Wash. ( TheStreet) -- A deadly explosion and fire at a Tesoro ( TSO) oil refinery is being probed by state and federal officials.

The U.S. Chemical Safety and Hazard Investigation Board sent investigators to the scene of the explosion, and the Washington Department of Labor and Industries started an investigation, according to an Associated Press report.

The blast at about 12:30 a.m. Friday killed five people, according to an AP report. Three men died at the scene, two women died later at a Seattle hospital, and two other people were badly burned and were in critical condition, the report said.

Anacortes is about 70 miles north of Seattle on Washington's Puget Sound.

Tesoro has started its own probe, the report said.

The refinery recently was fined for safety violations, the report added.

The explosion was the worst refinery accident since a 2005 blast at a BP ( BP) refinery in Texas killed 15 people and injured 170, the AP noted.

Shares of Tesoro closed Thursday up 49 cents at $14.39. U.S. stock markets were closed Friday in observance of Good Friday.

This article was written by a staff member of TheStreet.com.