Chantelle McDougall, 30, her cult leader boyfriend Gary Felton, 48, their daughter Leela McDougall, 10, and friend Tony Popic, 44, were last seen on July 13, 2007 in Busselton where they sold a car for $4000 to a local dealer and drove away in a waiting vehicle.
The group, none of whom have touched their bank accounts since, told family and friends they were headed for Brazil.
Four days later, Tam Airlines domestic flight 3054 from Porto Alegre to Sao Paulo crashed at Sao Paulo Airport, killing 181 passengers, six flight crew and 12 people on the ground.
The plane careered off the end of the runway, cleared a highway bordering the inner-city airport, slammed into a fuel depot and burst into flames.
The resulting heat was so intense that more than 70 of the bodies were so badly burnt they were either never recovered or could not be identified.
_The West Australian _understands the WA Police missing persons squad has been liaising with Brazilian authorities in an effort to determine whether the missing group were among the victims.
While the flight's passenger manifest is publicly available, and does not contain the names of any of the group, the issue has been complicated by Mr Felton's history of forging identity documents.
While living in Australia, where he operated a secretive doomsday internet forum, the Englishman went by the name Simon Kadwell - an identity he stole from a former British associate more than 15 years ago. It is understood WA detectives have been investigating whether the group could have travelled under false identities, but are yet to find any evidence that they did. Police have previously said they had not left Australia under their real names.
WA Police refused to comment on the case yesterday.
Chantelle's father Jim McDougall said yesterday he had conducted his own investigations into the plane crash and did not believe the group were on board.
He did not believe there was enough time for them to have made it to Porto Alegre in time for the flight.
"We spoke to police about that crash a little while back but we haven't had any recent update on what they've come across," Mr McDougall said.
"We looked at the names on the passenger list and didn't find theirs, which was a big relief.
"We strongly believe they are alive and are hoping that they will make contact sooner, rather than later."
Mr McDougall and his wife have previously accused Mr Felton of brainwashing and seducing their daughter when, as a 17-year-old, she started babysitting for him.
Mr Felton and Chantelle had Leela and in 2004 moved with Mr Popic to Nannup, where Mr Felton operated the doomsday forum called The Gateway.He was called Si in the internet chat forum, which involved about 40 members around the world who referred to themselves as the Forecourt - a religious reference to the place where believers wait for "judgment day".