ROBERTS, Wis. (AP) – Federal investigators say a reward is being offered for information about this week’s arson at a western Wisconsin school that caused $1.5 million in losses.  The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives says a reward of up to $5,000 is being offered for information leading to the arrest of whoever set the fire at St. Croix Central Elementary School in Roberts.  The fire was reported around 3:45 a.m. Tuesday. Investigators concluded the fire was arson.  No one was hurt. Classes were canceled at the elementary school for the rest of the week but will resume Monday at an alternate site.  Anyone with information can call the St. Croix County sheriff’s department at 715-386-4701 or the ATF’s Madison field office at 608-441-5050.