A small airplane hit a bald eagle before it crashed and burst into flames just north of Anchorage last month, killing all four people on board, authorities said Wednesday.

It is the nation's first civilian plane crash to result in deaths after an impact with a bald eagle, said Shaun Williams, a National Transportation Safety Board investigator. There have been other crashes involving eagle strikes that resulted in serious injuries, he said.

The pilot, co-pilot and two passengers died when the plane went down April 20 near a small airport about 20 miles north of downtown Anchorage. They were conducting an aerial survey for a private firm.

Investigators found an unidentified substance on several portions of the plane's frame and sent samples to the Smithsonian Institute in Washington, D.C., for forensic analysis.

"There, they were able to determine that the portions of feather and other material came from an immature bald eagle," Williams said.

Killed in the crash were the pilot, George Kobelnyk, 64; co-pilot, Christian Bohrer, 20; and two passengers, Sarah Glaves, 36, and Kyle Braun, 27. The pilot was formerly with the NTSB and retired from the Federal Aviation Authority, Williams said.

The four were taking aerial photographs from an area near the Birchwood airport to the northern part of Cook Inlet.

Much of the wreckage was found in an area of dense spruce and birch trees. A post-crash fire consumed most of the fuselage.

The Alaska Department of Fish and Game's website says Alaska has the largest population of bald eagles, which are found only in North America. It puts the Alaska bald eagle population at about 30,000 birds.