Federal safety officials are investigating the April 16 collapse of a trench that killed two construction workers in northern Colorado. The two men—Cristopher Lee Ramirez, 26, and Jorge Baez Valadez, 41—were installing utilities at a site being developed by D.R. Horton Express Homes in Windsor, Colo., when they were trapped by soil and rocks in the 15-ft-deep trench. The rescue attempt lasted seven hours and involved small shovels because of fears of a second collapse.