Two people were injured, one of them seriously, in an accident at a power plant that burns wood chips and other waste to produce electricity.
Amador County Undersheriff Jim Wegner said investigators think "catastrophic mechanical failure" caused a boiler at the Buena Vista Biomass Power plant to rupture with such force Wednesday morning that workers initially thought there had been an explosion.
One of the injured was airlifted to a Sacramento hospital with burns and cuts. The other was treated at the scene.
The 18-megawatt plant about 57 miles southeast of Sacramento used to run on coal, but was converted to biomass last year.
An environmental group sued two years ago to block the project over concerns that nearby forests would be harvested as fuel. A settlement called for the company to report the sources of its materials.