A federal jury on Friday acquitted W.R. Grace & Co. and three of its former officials of charges that they knowingly exposed residents of Libby, Mont., to asbestos poisoning associated with a mining operation and conspired to hide it. The verdict brings to an ignominious end one of the most significant criminal prosecutions the government had ever filed against a corporate polluter. The acquittals raise new questions about prosecutorial failings in the Justice Department, which already was reeling from the dismissal of its corruption case against former Sen. Ted Stevens (R-Alaska).In Libby, where an estimated 1,200 residents have died or...