Mexican authorities have revealed chilling details about the killings of Perth brothers Jake and Callum Robinson and their American friend, after examining three bodies found in a deep well.

Police said all three victims appeared to have died from a single gunshot wound to the head, Channel Seven reports on Monday morning (AEST).

It’s believed the Robinsons and their travel mate Carter Rhoad were the victims of a bungled carjacking while on their surfing trip in a remote region of Baja California.

A preliminary hypothesis suggests three Mexicans, who have been arrested, attempted to steal the tourists’ white Chevrolet ute.

When the surfers resisted they were fatally shot, with their bodies dumped in a 15-metre well on a cliff-top in the Baja Peninsula, on Mexico’s west coast.

“When they tried to get the vehicles, the victims opposed the robbery. The robbers were armed with a firearm and then apparently shot the victims,” Baja California Attorney General Maria Elena Andrade Ramirez said.

A local laboratory will run forensic tests to confirm if the bodies discovered on Saturday are the Perth siblings and the American.

A fourth body was discovered in the well, although that corpse had been there for longer and was not believed to be linked to this case, Andrade said.

Local news outlet Zeta reported the fourth body was believed to be that of the property’s owner, who had been missing for weeks.

The chief state prosecutor’s office said evidence found along with abandoned tents at a burnt-out campsite was linked to the three people being questioned about the missing foreigners.

Investigators also found a burnt-out white ute, believed to be the vehicle in which the men were travelling when they disappeared.

All three Mexicans have been charged with forced disappearance, which is equivalent to kidnapping.

The Robinson brothers, both in their 30s, were travelling in the region on a surfing holiday when they failed to check into pre-arranged accommodation near the city of Ensenada.

Perth siblings Callum and Jake Robinson went missing on a surfing and camping trip.

‘Understand the risks’

Meanwhile, a long-time local and editor of Talk Baja, Ron Gomez Hoff, outlined the dangers of camping in remote locations of Baja state where cartels and other criminals are likely to operate.

“Bottom line is, it’s a 50/50 chance that within an hour’s time of first setting up camp, someone will be there behind the rocks and the bushes, quietly watching your movements from a distance, sizing you up and making mental notes of your possessions and vulnerabilities,” he warned in an open letter.

“It’s the simple reality today in remote areas where even if you believe you are alone, most likely somebody saw you drive in along the way. If a cartel is operating in the area, they will most certainly be notified and kept aware of any movement on your part in their direction.

“Whether or not they decide to actually try to make a move on your camp is difficult to predict and mostly based on how far away to the nearest town/cell signal, the number of men in your group compared to their numbers, if they have a gun and whether or not they think one of you might have a gun, what types of drugs they are using and how desperate they are.

“If you happen to set up camp near a cartel operation involving moving people or drugs nearby, they may just stake lookouts around your camp to ensure that nobody from your group approaches, threatening their activity.

“And hopefully nobody does.”

Gomez Hoff urged tourists to follow the lead of locals and avoid isolated, remote areas.

“The locals don’t take their families to explore or camp in Baja’s most remote areas these days,” he said.

“Most Mexican families down here today who want to camp prefer to seek out well-travelled, popular camping areas with lots of people.

“They know and understand the risks, as well as the safety in larger numbers. They have to deal with the realities of crime down here on a daily basis, not for just a few days a year while on vacation.”

Rescue workers at the scene where the human remains were found near La Bocana Beach. Photo: AAP

Last week, Debra Robinson appealed for help to find her two sons and their US friend Rhoad, saying she had not heard from them since Saturday.

“Callum is a type one diabetic so there is also a medical concern,” she wrote on Mexican social media.

Officials from the Department of Foreign Affairs and Trade said it was in contact with the family and it was obviously a distressing time for them.

Foreign Affairs Minister Penny Wong reiterated that the government was doing everything it could to support the family

“I can indicate that our embassy in Mexico, as well as the Australian Federal Police, are working in support of local authorities, that my department is providing consular support to the families concerned,” she said on Saturday.

Health Minister Mark Butler said the department was trying to glean as much information as possible from Mexican authorities to inform the family.

“This is deeply distressing to the Australian community, particularly the family and friends of these two young Australians” he said on Saturday.

The brothers’ family is understood to be travelling from Perth to Mexico to be closer to the investigation.

The US State Department is also monitoring the investigation.

US and Australian authorities have urged people to be particularly cautious when travelling to Baja California “due to the threat of violent crime”.

-with AAP