Coleman and Dean Lucas were traveling from Edmonton in Canada to Mexico and failed to arrive as planned on Nov. 21 in the city of Guadalajara.
Guadalupe Martinez, a spokesman for Higuera`s office, said the bodies were so badly burned that only tests like DNA or dental records could identify them. He said those tests were being carried out.
Higuera said the two Australian surfers got off a ferry from the Baja California peninsula at Topolobampo, Sinaloa, at about 10:30 p.m. on the night of Nov. 20.
They apparently began driving immediately, but didn`t get far.
The van and the bodies were found a day later in a rural area of Navolato, about 100 miles (160 kilometers) south of Topolobampo, after police received a citizen`s report on the abandoned vehicle.
Higuera said "they may have camped out on the way to Guadalajara."
Sinaloa is a drug gang-plagued state that is home to the cartel of the same name.
Lucas` partner, Josie Cox, said she`d received a text message the night of Nov. 20, and she said Coleman had planned to meet his girlfriend, Andrea Gomez, in Guadalajara, on Nov. 21.
"Adam wanted to get to Guadalajara as soon as possible," Cox said. "I presume that from when they got off the ferry to Guadalajara, it was an unfortunate time ... and they maybe pulled in to sleep or something."
An online fundraising campaign to support the two men`s families to travel from their home in Golden Bay in Western Australia state had raised more than 22,000 Australian dollars ($16,000) within a day.
Family members will travel to Mexico within days to assist Mexican police and Australian consular officials. A GoFundMe page has received donations from almost 230 people since it was established late on Sunday.
"My brother`s family is overwhelmed by the response," Coleman`s aunt Jane Cattermole said on Monday.
"Now they can bring Adam home," she added.
Australian Foreign Minister Julie Bishop said the government is working closely with Mexican law enforcement.
"But our thoughts are with the families and friends of those two men who have been missing for some time, but I do hold very grave concerns for their fate," Bishop told Nine Network television in Canberra, Australia.
Coleman`s mother, Zena Cattermole, wrote on her Facebook page: "Our son`s life has left us, but they will be with us forever in our hearts."
In 2014, another tourist was killed along Mexico`s scenic but dangerous Pacific coast.
Harry Devert, a 33-year-old stock trader from Pelham, New York, vanished while heading on his motorcycle to Brazil for the World Cup.
His decomposed remains were found along with his motorcycle months later much further south on the coast, near the resort of Zihuatanejo.
Mexico`s federal police later arrested Adrian Reyes Cadena, the purported leader of a drug gang in Zihuatanejo, and implicated him in the killing.
A federal official said Reyes Cadena`s gang apparently thought Devert was a U.S. agent.
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