A plane that went down during a survey flight near the Birchwood Airport last month, killing four, struck a young bald eagle before it crashed, according to the National Transportation Safety Board.

The agency announced the new findings Wednesday on the April 20 crash that killed 64-year-old former NTSB investigator and pilot George Kobelnyk, as well as passengers Christian Bohrer, 20, Sarah Glaves, 36, and Kyle Braun, 27. A preliminary report released last week said the Cessna 172 took off from the airport shortly before 9 a.m. and made several low turns in the area before crashing and bursting into flames in a densely wooded area off Beach Lake Road.

"During the course of the investigation, which is still ongoing, a foreign substance was discovered on the airframe of the accident airplane," NTSB officials wrote. "A forensic analysis of the substance was completed by the Smithsonian Institution in Washington, D.C., and it was determined to contain feather particles of an immature bald eagle."

Shaun Williams, the NTSB's lead investigator on the crash, said eagle residue was recovered from various points on the plane.

"There were several areas on the tail structure of the aircraft" where the material was found, Williams said.

He said it wasn't clear whether the bird entered the plane's cabin.

"That's hard to determine at this point, due to the post-crash fire and extensive thermal damage," Williams said.

The new information about the April 20 crash makes it the first of its kind in the U.S., according to the NTSB.

"It's the first civil aircraft accident following an impact with a bald eagle that resulted in fatalities," Williams said.

Bald eagles have been involved in at least two other aviation incidents in the state, both involving Alaska Airlines jets flying in Southeast Alaska. In September 2003, a flight from Seattle struck an eagle while on approach to Petersburg, and in April 1987 a flight leaving Juneau en route to Yakutat saw an eagle drop a fish onto a cockpit window during takeoff. Both flights were subsequently delayed by damage inspections.

Marc Pratt, the district supervisor for the U.S. Department of Agriculture's Wildlife Services Program, said Wednesday his office works with state and federal agencies to keep birds away from civilian and military airports across Alaska. Federal protections can complicate the job where eagles are concerned, but Pratt said the USDA can remove items like beached fish carcasses that might lure eagles to the area, as well as train airport staff on methods to haze or mitigate wildlife.

In some cases, Pratt said, the U.S. Fish and Wildlife Service has approved removing unoccupied eagle nests from airport grounds.

"We're not trying to keep them from nesting; we're trying to keep them from nesting on airport property," Pratt said. "We're hoping that nesting pair will move into an area that's not in a flight corridor, not in the path of incoming aircraft -- it's safer for the eagles and safer for aircraft."

At the NTSB, Williams declined to say whether the bird strike affected control of the Cessna during the deadly flight, noting the investigation is still in its infancy.