Federal antitrust investigators are scrutinizing the Denver-based Alterra Mountain Company’s pending acquisition of Arapahoe Basin, one of Colorado’s last ski areas independent of corporate resort empires. Alterra officials characterized it as a “customary” review that the U.S. Department of Justice requires for large transactions. The investigators have asked for business data on skiers at resorts across the Rocky Mountain region from the National Ski Areas Association. Department of Justice officials were mum.