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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A man suspected of planning a terrorist attack has been arrested in Rotterdam after police found a loaded AK-47 in his apartment.
Another mild day with highs reaching up to 13C degrees in mild southerly winds
The first officer shouted "look!" as the drone passed next to the right wing at around 11,000ft above south-east London.