(CNN) -- The National Transportation Safety Board said Monday that a bird strike may have preceded the January 4 crash of a Sikorsky helicopter near Morgan City, Louisiana.
The crash killed eight of the nine people aboard.
Though a bird specialist's initial visual examination found no evidence of a bird strike, DNA testing of a swab taken from the windscreen on the pilot's side of the Sikorsky S-76C++ helicopter found microscopic remains of a hawk, the NTSB said.
"The swab was taken from an area of the windscreen that exhibited concentric ring fractures," the NTSB said in a written statement. "Similar concentric rings were visible in the gel coat of the fuselage area just above the windscreen."
Last week, in a second examination of the wreckage, small feather parts were also found "under a right-side windscreen seal and in the folds of the right-side engine inlet filter," the statement said.
More swabs for bird remains were taken from other parts of the helicopter, which was operated by PHI Inc., it said.
The craft's original windscreens had been replaced by PHI about two years ago "as part of their normal procedures," the statement said.
They had been replaced for a second time about a year before the accident with a lighter-weight, acrylic windscreen that is approved by the Federal Aviation Administration, the statement said.
Further investigations into possible scenarios that could have led to the crash are planned, it said.
Birds are a known hazard to flyers. The NTSB reported two weeks ago that it was Canada geese in both engines that forced US Airways Flight 1549 to ditch into the Hudson River last month.
The flight crew of the Airbus A320 put the plane down gently on the river after the bird ingestion caused both engines to lose power. All 155 people aboard survived.
Investigators said bird remains -- called snarge -- were found in both turbofan engines, which were certified in 1996 as able to withstand bird ingestion up to 4 pounds. Almost all adult Canada geese weigh more.
Investigators have not determined how many birds hit the jet's engines.