NEWBERG, Ore. -- Investigators have concluded that the two small planes which collided over Newberg in October of 2011 crashed because the pilots could not see one another, according to the National Transportation Safety Board.

Flight instructor Travis Thompson, 31, of Beaverton and Henrik Murer Kalberg, 23, of Hillsboro walked away after crash-landing their plane in a field near Newberg. Steven Watson, 58, of Beaverton, was in the other, smaller plane. He died after his Bonanza single-engine aircraft broke apart and crashed to the ground, authorities said.

The NTSB just released its final report on the investigation into the deadly crash. It did not assign blame for the crash and determined that The pilot was unable to see the other aircraft to avoid a collision.

Thompson's twin-engine plane was doing a series of training ascents and descents and it clipped the single-engine plane, Capt. Ken Summers, with the Yamhill County Sheriff's Office, said on the day of the crash. Witnesses also told KGW that the back end of the smaller plane was sheared off, sending it plummeting to the ground.

NTSBinvestigators said the pilots' views were obstructed by a wing, engine, door post and cabin roof.

Thompson was an experienced flight instructor. Watson was a retired Oregon State Police commander who flew as a hobby.

More: Pilot Steven Watson's career, family

Investigators said the mid-air collision happened in an area not under formal air traffic control and the pilots were flying by visual flight rules.

Audio: Listen to the pilot's radio transmission

Watson had taken off from a small airport in McMinnville. The twin-engine plane had taken off from the Hillsboro Airport.

Investigators said when pilots are flying by visual flight rules and doing training maneuvers, it is the responsibility of the pilot performing the maneuvers to make sure the area is clear.

About 50 people were camping Champoeg State Park, near where the debris landed, but no one was hurt. They were all evacuated after the crash.

More: Plane crash debris nearly hit campers

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