A 16-year-old student -- who authorities say opened fire at his Colorado high school on Wednesday, wounding two classmates and then turning the gun on himself -- was believed to be "radicalized by some extremist network,” officials with the local sheriff's office said.

At least two students have been wounded in a shooting at a suburban Denver high school, authorities said Wednesday.

For nearly three decades, Colorado has experienced some of the highest-profile shootings in the nation – tragedies that claimed the lives of ordinary Americans sitting in classrooms or movie theaters...