This is a breaking news story. Check back for updates.
UPDATE 3:57 p.m. EST: Arapahoe County Sheriff Grayson Robinson said the shooter -- a student -- is dead from an apparent self-inflicted gunshot wound. One victim was transported to the hospital in serious condition, while a second victim suffered a "minor wound," although authorities are unsure whether the injury was from a gunshot wound.
UPDATE 3:48 p.m. EST: One student is in critical condition following the shooting at Arapahoe High School, a hospital spokesman told the Associated Press.
UPDATE 3:26 p.m. EST: According to breaking news Twitter account @PzFeed, shots were fired inside the cafeteria of Arapahoe High School. Schools in Elizabeth, Colo., are also under lockdown following the incident.
UPDATE 3:11 p.m. EST: Littleton Public Schools are under lockdown in response to the active shooter situation at Arapahoe High School.
An "active shooter" has shot at least two people at Arapahoe High School in Colorado.
Two people have been injured at the Centennial, Colo., high school, according to Denver television station KUSA.