REDDING, Calif. (AP) — Authorities are investigating a water tank explosion that killed a worker at a sewage treatment plant in Redding.

The Shasta County coroner's office says 54-year-old Fred Crumb died Thursday afternoon after suffering critical head injuries in the explosion.

The Redding Record-Searchlight reports that Crumb was critically injured when the explosion blew the top off a 4,500-gallon steel tank at the Clear Creek Wastewater Treatment Plant.

The city is in the process of upgrading and expanding the facility.

Redding Public Works Director Brian Crane says the city is investigating what caused the 33-year-old tank to explode.

The California Occupational Safety and Health Administration has also launched an investigation.

Crane says the explosion interrupted the plant's treatment operations for about two hours.