MORGANTOWN, W.Va. (AP) — Federal prosecutors investigating the West Virginia mine explosion that killed 29 men widened their net Wednesday, filing criminal charges against a longtime Massey Energy executive who worked closely with former CEO Don Blankenship — a move signaling where their sights may be ultimately set.

The cooperation of David Craig Hughart is a sign that authorities may be gathering evidence to target officials further up the Massey hierarchy. Some victims’ families hold Blankenship personally responsible for the worst U.S. mining disaster in four decades, though prosecutors have declined to say who else could face charges in the wide-ranging and continuing probe.