After firing a round down a hallway, Karl Pierson, 18, shot a fellow student who happened to be sitting nearby with a friend as he headed toward the library.
Claire Davis, 17, was shot in the head at point-blank range and remained in hospital in critical condition yesterday.
Pierson set off one of the devices but killed himself just one minute and 20 seconds after entering the building because he knew a sheriff's deputy assigned to the school was closing in, Sheriff Grayson Robinson said at a news conference.
Pierson's original target was believed to be a librarian who coached the school's speech and debate team. The librarian, whose name was not released, had disciplined the teenager in September for reasons that have not been disclosed.
The librarian was able to escape the school unharmed, Mr Robinson said.
The scene unfolded on the eve of the anniversary of the December 14, 2012, massacre at Sandy Hook Elementary School in Newtown, Connecticut, in which a gunman killed 20 children and six staff.
Arapahoe High School is just eight miles from Columbine High School, where in 1999 two teenage gunmen killed 12 classmates and a teacher before killing themselves. More recently, a gunman killed 12 people at a cinema in Aurora, another Denver suburb, in the summer of 2012.
The sheriff also said Ms Davis appeared to be a random target, adding: "His evil intent was to harm multiple individuals."