Baja California state prosecutors released grisly details of the slayings, but have not yet officially confirmed the identification of the bodies.
Authorities in the Baja California state are conducting forensic tests on three bodies they found in a well this week and which they believe to be Australian brothers Callum, 33, and Jake Robinson, 30, as well as American Carter Rhoad, 30.
Parents of the presumed victims have arrived in the country and will try to identify the dead bodies, a Mexican regional official said on Sunday.
The corpses were decomposing after the thieves dumped them into a remote, 15-metre well. If relatives can't identify them, further tests will be conducted. The well also contained a fourth cadaver that had been there much longer.
"The probability that it's them is very high," said chief state prosecutor María Elena Andrade Ramírez said.
The three men went missing last weekend during a camping and surfing trip, posting idyllic photos on social media of waves and isolated beaches along a stretch of coast south of the city of Ensenada.
But Andrade Ramírez described the moments of terror that ended their trip.
She said the killers drove by and saw the foreigners' pickup truck and tents, and wanted to steal their tyres.
"Surely, they resisted," she said of the victims, and the thieves shot them to death.
The thieves then went to what she called "a site that is extremely hard to get to" and dumped their bodies into a well they apparently were familiar with.
She said investigators were not ruling out the possibility the same suspects also dumped the first, earlier body in the well as part of their thefts.
"They may have been looking for trucks in this area," Andrade Ramírez said.
The site where the bodies were discovered near the township of Santo Tomás was near the remote seaside area where the missing men's tents and truck were found Thursday along the coast.
From their last photo posts, the trip looked perfect. But even experienced local expats are questioning whether it is safe to camp along the largely deserted coast anymore.
The moderator of the local Talk Baja internet forum, who has lived in the area for almost two decades, wrote in an editorial on Saturday that "the reality is, the dangers of travelling to and camping in remote areas are outweighing the benefits anymore".
Baja California prosecutors had said they were questioning three people in the case. On Friday, the office said the three had been arrested on charges of a crime equivalent to kidnapping, but that was before the bodies were found. It was unclear if they might face more charges.
At least one of the suspects was believed to have directly participated in the killings.
Last week the missing Australians' mother, Debra Robinson, posted on a local community Facebook page an appeal for help in finding her sons. Robinson said Callum and Jake had not been heard from since April 27. They had booked accommodations in the nearby city of Rosarito.
Robinson said one of her sons, Callum, was diabetic. She also mentioned that the American who was with them was named Jack Carter Rhoad, but the US Embassy in Mexico City did not immediately confirm that.
The US State Department said it was aware of reports of a US citizen missing in Baja, but gave no further details.
In 2015, two Australian surfers, Adam Coleman and Dean Lucas, were killed in western Sinaloa state, across the Gulf of California - also known as the Sea of Cortez - from the Baja peninsula. Authorities said they were victims of highway bandits. Three suspects were arrested in that case.
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