EATON RAPIDS, Mich. (AP) - Michigan fire investigators say they have failed to determine the cause of the initial explosion that led to a fire that severely damaged an auto parts manufacturing plant. Eaton Rapids Fire Chief Roger McNutt said Monday he concluded his investigation into the May 2 series of explosions at Meridian Magnesium last week. He added he considers the investigation over. After a first explosion at the plant, two more followed, causing about $4 million in damage to the factory that makes structural parts, a third of which goes to Ford. As a result, production of Ford's Super Duty at its Louisville, Kentucky, truck plant and the F-150 at its Kansas City, Missouri was disrupted. Fire officials initially said the fire at the Meridian Magnesium complex caused the explosions. McNutt said Monday explosions started the only fire, a blaze on the main plant's roof.