Two Aussie surfers are believed to have been intercepted by highway bandits masquerading as police before meeting a grisly end on a quiet back road, say Mexican authorities.

While it will be at least another week before authorities can confirm if two burnt bodies found in a van in Mexico belong to the Australian mates, it is now believed Adam Coleman was shot in the face when he resisted a robbery as they travelled through the infamously dangerous Sinaloa state.

Mr Coleman and Dean Lucas, 33, were on their way to Guadalajara after midnight on November 21. The van they were travelling in was found nine days later destroyed by fire off a dirt road in a deserted rural area near the town of Navolato, with two bodies inside.

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Sinaloa State Attorney General Marco Antonio Higuera said last Friday they have detained three suspects in connection with the presumed killing of two Australian tourists who disappeared on November 20. Photo: AAP

Authorities do not believe it was an opportunistic robbery, but was planned, and that the bandits, dressed in police uniforms, had staked out a section of the Benito Juarez Highway in the early hours of the morning.

Investigators now say both were still alive, although Mr Coleman was likely seriously injured, when they were driven from the highway under the cover of darkness.

Prosecutors do not believe they were restrained.

“One of them was severely injured,” Guadalupe Martinez, spokesman for Sinaloa state’s attorney-general, told AAP on Monday.

“I imagine the other one was too scared to do anything.”

When they reached the rural area the robbers allegedly shot the Australians and torched Mr Coleman’s old Chevy van, with their bodies inside.

Two bodies burnt beyond recognition were found inside. DNA testing, involving comparing DNA from the two Australians’ family members with the charred bodies, is yet to be completed.

“We haven’t picked the Australian DNA samples up yet. They haven’t been identified,” Mr Martinez said.

Their burnt-out van was found on November 30. Photo: AAP

“It will take at least another week.”

Sinaloa authorities have arrested three of the alleged bandits – Julio Cesar Muniz, Martin Rogelio Muniz and Sergio Simon Benitez – and continue to pursue others.

They say the investigation is complete, with just the final arrests to be made.

“We are looking for the person for which we have an arrest warrant and getting warrant for all the other ones,” Mr Martinez said.

“We have a group of officials – search officials – because the investigation is complete.”

Embarking on a Mexican surfing adventure

The Perth pair were travelling from Edmonton in Alberta, Canada to Guadalajara to meet Mr Coleman’s girlfriend on November 22.

Adam Coleman loved surfing, travel and laid-back fun. Photo: AAP

They were reported missing after they failed to arrive.

The duo grew up together in the Perth suburb of Golden Bay, before moving to live and work in Broome in the early 2000s.

Mr Coleman fell in love with Mexican girl Andrea Gomez during a surf trip in February, and he and Lucas decided to return later in the year to visit her in her hometown.

They spent their final days travelling on a ferry from Baja California, disembarking at night on November 20 before beginning a journey toward Guadalajara, some 900 kilometres away.

They were last seen later the same night in a 24-hour convenience store buying a map, before the gruesome discovery on November 30.

– with Peter Mitchell, AAP