A pair of overalls could have contributed to the helicopter triple fatality in Wanaka last month.
Investigators remain tight-lipped about the October crash which killed pilot Nick Wallis, along with DoC rangers Paul Hondelink and Scott Theobald.
But the New Zealand Herald understands they are following strong leads over a possible contributing cause.
It's believed the occupants of a trailing helicopter, on the same tahr cull operation, saw clothing being sucked out of the machine and wrapping around the tail rotor.
The debris then caused the machine to become unbalanced in flight.
The tail section broke away before the aircraft crashed into an open paddock near the Clutha River and burst into flames.
Investigators are refusing to comment...
But Nick Wallis' older brother Jonathan says the family's confident they'll find out what happened.
The wreckage and debris were removed from the crash site within days and taken to Wellington for forensic examination.