CNN reports that Sheriff Grayson Robinson told reporters that at least two people have been injured at the school, but there has been no word on if the injured parties are students, teachers, or even belonged at the school.
Officials are reportedly racing to the scene where police say the shooter may still be inside the high school, but that fact has not been confirmed.
Reports indicate that the schools in the Littleton Public Schools system have been placed on lock down until further notice.
Arapahoe High School is in Centennial, Colorado, which is 15 miles south of Denver. It is reportedly the largest high school in the Littleton Public Schools system with more that 2,200 students enrolled.
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