ANCHORAGE, Alaska — A small airplane hit a bald eagle before it crashed just north of Anchorage, Alaska, last month, killing all four people on board.

An investigator says it’s the nation’s first civilian plane crash to result in deaths after an impact with a bald eagle.

Shaun Williams with the National Transportation Safety Board says there have been other crashes involving eagle strikes that resulted in serious injuries, not deaths.

The pilot, co-pilot and two passengers died when the plane went down April 20 near a small airport about 20 miles north of Anchorage.

Williams says an unknown substance was later found on the aircraft. Analysis at the Smithsonian Institution in Washington, D.C., determined some of it was feathers and other materials that came from an immature bald eagle.