GAYLORD, Mich. (AP) _ A second explosion ripped through a mill that produces particle board Saturday, injuring six firefighters a day after an initial explosion injured nine people, at least five critically.

One firefighter suffered first- and second-degree burns, and five others were treated for smoke inhalation and minor injuries after the blast about 12:30 a.m., said David Duffield, chief of the Otsego County Fire Department.

A seventh firefighter suffered a knee injury while fighting the blaze started by Friday's explosion at the Georgia-Pacific Corp. plant, Duffield said. All the firefighters were treated and released later Saturday morning.

Friday's midmorning blast occurred in an area where glue is mixed with wood chips, manager Joe Sims said, describing it as his ``worst nightmare.'' The 45 employees working at the time were evacuated.

Four of the victims, who were airlifted to various hospitals, were in critical condition Saturday morning. Another was upgraded to serious condition. Others were treated for shock and released.

The cause of the blast was not immediately clear. State fire and safety officials were investigating, and Georgia-Pacific dispatched safety inspectors to the scene.

The plant employs about 230 workers and is one of the top five employers in Otsego County, a mostly rural area 200 miles northwest of Detroit, city manager Joe Duff said. The plant is in an industrial area just outside Gaylord, population 3,600.