SALT LAKE CITY (AP) — Federal investigators say a plane that slammed into a Utah highway started making unusual sounds during takeoff and apparently struggled to stay at altitude before the crash that killed four people.

The National Transportation Safety Board spokesman Eric Weiss said Thursday the exact cause of the crash last week could take more than a year to determine, but the agency has released early findings from the scene. The report states flight mechanics at the municipal airport in Ogden noticed the tail was bobbing as if the pilot was struggling to keep the craft at altitude on July 26.

Investigators say pilot Layne Clarke radioed air traffic control shortly after takeoff and said the plane was going down. The controller cleared him for landing, but the small plane hit the ground four seconds later.

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