Papua New Guinea's Civil Aviation Authority says it has completed its report about a jet that landed illegally at Bougainville's Aropa airport.
The deputy director, Wilson Sagati, says the report remains confidential as it is a sensitive police matter.
But he says more investigation is needed to determine why the aircraft landed at a decommissioned airport.
He says the matter is very serious.
"Pilots do contravene the regulations if you don't land at an authorised landing area, you're guilty of an offence if you do that under the Civil Aviation Act. If an aeroplane is in an emergency, that's a different matter, but this was not an emergency landing, it was a normal operation."
Wilson Sagati of PNG's Civil Aviation Authority.
Police have issued a warrant for the arrest of two passengers who vanished after the illegal landing - an Australian Jeff Richards and a Briton, James Nessbit.
