Australian brothers Jake and Callum Robinson confirmed dead in ...

Mexican authorities have positively identified the bodies of three men found in a well in the country's north as two Australian brothers Jake and Callum Robinson and their American friend Jack Carter Rhoad.

The parents of Jake and Callum visited the morgue in the Mexican city of Ensenada to identify their sons' bodies earlier on Sunday, local time. 

Police believe the trio were shot dead by thieves who were trying to steal their car for its tyres.

Officials revealed their early theory about the deaths as members of the surfing community in Ensenada, in Mexico's north-west, took to the streets to demand improved security in the area.

Three people remain in custody over the disappearance of the Robinsons, originally from Perth, and Mr Rhoad, who were camping in Mexico's north-west.

Their remains were recovered from what had been described as a disused well in the area on Friday. 

All three died from a single gunshot to the head, according to the Baja California Forensic Medical Service.

Callum and Jake Robinson disappeared while camping on Mexico's west coast.(Instagram: @Callum10Robinson)

That has raised suspicions locally about the possible involvement of drug cartels, which are rife in Baja California.

But the attorney-general for the Mexican state of Baja California, María Elena Andrade Ramírez, told a press conference on Sunday, local time, that authorities' early theory is that the men were killed in a robbery gone wrong.

She said it appears the attackers were passing through the area when they decided to steal the campers' ute, which they wanted for its tyres, and they shot the campers when they resisted.

"They may have been looking for trucks," she said.

"The perpetrators drove by and saw the foreigners' ute and tents, and wanted to steal their tyres.

"The perpetrators approached with the intention of taking over the ute to remove the tyres, and put them on their older car. It is likely that there was resistance on the part of the tourists, and the aggressors took out a weapon in their possession." 

On Sunday afternoon, an estimated 500 people marched through Ensenada, chanting in Spanish, "We want security".

People marched in Ensenada, including one holding a surfboard that said, in Spanish, "Australia we are with you".(ABC News: Carrington Clarke)

Some held surfboards with messages including "Australia, we are with you" and "They only wanted to surf, and they executed them".

Local surfer Héctor Estrada said he was marching to call for justice for the men, as well as measures to improve safety at local beaches.

"We are here in solidarity with our friends from Australia and the United States," he said.

"We are a big community of surfers all over the world and we need security, we need the beaches to be safe [for] camping around, surfing or just playing with your family, friends."

Hundreds marched in Ensenada.(ABC News: Carrington Clarke)

Another local, Jorge Emilio Martinez, said the community was outraged over the suspected murders.

"We feel profound shame and sorrow that this happened in our country," he said.

"We are a country that loves receiving people from all over the world … this is not who we are as a country."

Ana Acosta said it had been a rough time for the local surfing community.

Ana Acosta (left) said the government needed to do more to keep surfers safe.(ABC News: Carrington Clarke)

"I'm asking for justice and security from the government and for the state to provide to us surfers," she said.

"You never hear about one girl just going out in the water and surfing, that would be very risky. I want future generations to have the security of going to the ocean and feeling safe."

A fourth body was found at the same location as the three suspected to be the campers. But it appeared to have been there for much longer and was not thought to be related, Ms Andrade Ramírez said.

A 23-year-old woman, her partner and his brother were detained last week by Mexican authorities.

One of the men, named as Jesús Gerardo García, now faces a "forced disappearance" charge. He has a history of drug dealing, domestic violence and vehicle theft, Ms Andrade Ramírez said.

Jesús Gerardo García faces a "forced disappearance" charge.

His brother and the woman are in custody for possession of methamphetamine.

The woman was allegedly found with a mobile phone belonging to one of the Australian men.

The attorney-general said investigators were confident more than three people were involved in the attack.

She could not rule out a connection to organised crime. 

"We cannot rule out the possibility that there are additional individuals who were involved in this incident," she said.

This demonstrator's surboard says, "They only wanted to surf, and they executed them".(ABC News: Carrington Clarke)