CHEYENNE, Wyo. (AP) Investigators say the pilot of a small plane radioed that he couldn’t maintain altitude because of mountain wind currents shortly before the plane crashed in the Wyoming mountains, killing all four people aboard.

A preliminary report from the National Transportation Safety Board released this week also says the pilot reported that a trace of ice was forming on the plane and that he was encountering light turbulence.

The crash killed the pilot, 40-year-old Luke Bucklin of Minneapolis, his 14-year-old twins Nate and Nick and his 12-year-old son Noah.

The plane crashed Oct. 25 in the Wind River mountains. It took searchers a week to find the wreckage.

The NTSB says Bucklin took off from Jackson at about 1 p.m. and crashed about 50 minutes later.

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