Clear skies and calm weather preceded deadly US Army helicopter crash in Alaska.

The mid-air collision of two US Army helicopters in Alaska that killed three soldiers and injured one happened in clear skies and calm weather, according to a report. The cause of the crash remains unknown, and investigators are planning to fly to the accident scene. The Army has grounded its aviation units for training following the crash and a Black Hawk helicopter accident in Kentucky that killed nine in March. The safety of aviators is the top priority, said Army Chief of Staff James McConville.