From The Missoulian, "Prosecutors dismiss charges against final Grace defendant":

Last month, Grace and three individual defendants - Robert Bettacchi, Jack Wolter and Henry Eschenbach - were acquitted of charges relating to a criminal conspiracy involving Clean Air Act violations and obstruction of justice. A fourth defendant, the company's in-house legal counsel, O. Mario Favorito, was severed from the case due to potential conflicts with his defense, and he was slated to stand trial separately in September.

However, prosecutors decided to forego efforts to bring Favorito's case to trial, apparently because of the not guilty verdicts. The jury of six men and six women heard 35 days' worth of testimony over the course of several months before finding the defendants innocent on all counts.

The Missoulian quotes the government's order, which is not yet posted at the court's document site for the case.

Earlier POL posts on the trial here. Whatever the motivation, prosecutors overreached by attempting to turn a regulatory and civil matter into a criminal one.