Bald eagle head.
A small plane plunging to the ground, killing all four people on board, after colliding with a bald eagle north of AnchorageKevin Law

A plane crash near an airport north of Anchorage that killed all all four people on board occurred after the small craft struck a bald eagle, investigators have revealed.

It is believed to be the first-ever fatal civilian plane crash in the US linked to an impact with a bald eagle, said National Transportation Safety Board investigator Shaun Williams. It's not yet known, however, if the bird strike actually caused the crash.

The pilot, co-pilot and two passengers were killed when the Cessna 172 went down in heavy woods in April 2016 during an aerial survey.

Mysterious residue was found on the plane, particularly on the tail, Williams told ABC News. It was sent to the Feather Identification Lab at the Smithsonian Institute in Washington DC for identification.

The forensics lab determined that "portions of feather and other material came from an immature bald eagle", said Williams. Several other bald eagles were observed over the crash site and nearby.

Air traffic control data from the Federal Aeronautics Administration shows the plane making several sharp turns in the area of the airport before the crash. It is possible the pilot was attempting to dodge an impact with the big bird.

"We're still going back and try to review past flights to see how this flight path compared to previous flights," Williams said.

Alaska has the largest population in the nation of bald eagles, which are only found in the US. There are an estimated 30,000 of the birds in the state, according to the Alaska Department of Fish and Game.

Marc Pratt, the district supervisor for the US Department of Agriculture's Wildlife Services Program, said that his office works with state and federal agencies to keep birds away from civilian and military airports across Alaska.

In some cases, the US 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 told the Alaska Dispatch News. "We're hoping that nesting pairs 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."