The suspected shooter at Arapahoe High School may have been a student seeking revenge over a conflict with a teacher, Arapahoe County Sheriff Grayson Robinson said.
Sheriff Robinson said the identity of the gunman will not be released "until everything is confirmed".
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The Duke and Duchess of Cambrige have been together for 15 years and will have three children under the age of four.
News of William and Kate’s third child was announced on September 4 and the duchess went on maternity leave on March 22.
Prince Harry will move down to sixth in line to the throne.