Three suspects have been detained in connection with the presumed killing of two Australian tourists who went missing last month, prosecutors in Mexico's Sinaloa state said Friday.

State prosecutor Marco Antonio Higuera said the three men were robbing motorists on a stretch of highway leading south through Navolato, Sinaloa. He said two other suspected members of the robbery gang remained at large.

The three were arrested on low-level drug-dealing and weapons charges, but Higuera said he expects homicide charges to be filed against them soon.

"This is just a gang of five people who committed highway robberies. They don't have links to drug cartels," Higuera said. 

One of them had previously been investigated for the killing of a man in a similar robbery, prosecutors said.

The gang is alleged to have killed Adam Coleman and Dean Lucas after Coleman resisted the robbery just after midnight Nov. 21. The thieves allegedly shot the two Australians to death, then doused their van with gasoline and set it afire.

Two charred bodies found inside the van the next day have not yet been positively identified. Tests are continuing. But the van's vehicle identification number matched one registered to Coleman in Canada.

Evidence presented at a news conference showed the arrested men had rifles, a shotgun, pistols, 124 small bags of methamphetamine and jackets with police logos on them. It was unclear whether they were wearing the police jackets at the time they allegedly stopped the van.

According to prosecutors, a lookout for the gang spotted the van before it got to Navolato and advised his accomplices it was a likely target.

The thieves apparently stopped the van after it passed a toll booth, as the two Australians drove south through Sinaloa toward Guadalajara, Mexico's second largest city.

Prosecutors said the thieves, traveling in an SUV, forced the van to stop. Coleman purportedly struggled with the thieves, and one of them shot Coleman, but he didn't die immediately. The van was then driven to a rural road, where both victims were apparently shot to death and one member of the gang lit the van on fire.

Coleman and Lucas were traveling to Guadalajara from Edmonton, Alta., and failed to arrive as planned on Nov. 21. The two surfers got off a ferry from the Baja California peninsula at Topolobampo, Sinaloa, at about 10:30 p.m. on Nov. 20. The burned-out van was found the next day.