The bodies believed to be those of the two Australians and an American who went missing in the Pacific coast state of Baja California showed the three men were killed with gunshots to the head, Mexican authorities said on Sunday.

María Elena Andrade Ramírez, the state’s attorney general, said the families of the missing men had arrived in Tijuana to verbally identify the bodies. Authorities expected to have official confirmation shortly.

Callum and Jake Robinson, both in their 30s, were Australian siblings from Perth. Jake was visiting Callum, who lived in San Diego, California, for a planned surfing holiday, with their friend Jack Carter Rhoad, a US citizen who also lived in San Diego. The trio were reported missing when they failed to check in to pre-arranged accommodation in Rosarito, Mexico, last weekend.

The bodies were found dumped in a remote well about 15m deep, about 6km from the camping site where the foreigners were killed. If relatives could not identify the bodies, genetic tests would be conducted.

“The probability that it’s them is very high,” said Andrade Ramírez.

The preliminary hypothesis of the investigation is that the missing men were attacked by people who wanted to steal their car, partly because they wanted the tires.

“The attackers drove by [the travellers’ campsite] in their vehicle,” Andrade Ramírez said. “They approached, with the intention of stealing their vehicle and taking the tires and other parts to put them on the older-model pickup they were driving.

“Upon approaching and surprising [the missing men], there was surely resistance and these people, the attackers, took out a firearm they had and took their lives.

“When what was meant to be a robbery had got out of control, they tried to dispose of the bodies by throwing them into a well.

“They were not attacked because they were tourists, the intent was to steal their vehicle.”

The attackers had burned the tents and covered the well with boards, he said.

Journalists asked whether organised crime might have been involved, and whether the fact all were killed with shots to the head implied a kind of execution. Andrade Ramírez refused to speculate, saying that the car robbery hypothesis was what authorities had for now.

“The investigation has only just begun,” Andrade Ramírez said.

Three Mexican nationals have been detained, one of whom has been charged with kidnapping.

The other two are being held for possession of crystal meth, though Andrade Ramírez did not discard the possibility that they were linked to the crime.

“In fact, we are sure that more people took part in the attack,” he said.

At least one of the suspects was believed to have directly participated in the killings. In keeping with Mexican law, prosecutors identified him only by his first name, Jesús Gerardo, AKA “el Kekas”, a slang word that means quesadillas, or cheese tortillas.

Dozens of mourners, surfers and demonstrators gathered in a main plaza in Ensenada, the nearest city, to voice their anger and sadness at the deaths.

“Ensenada is a mass grave,” read one placard carried by protesters. “Australia, we are with you,” one man scrawled on one of the half-dozen surfboards at the demonstration.

A woman held up a sign that read “They only wanted to surf – we demand safe beaches.”

The men had planned a camping trip near the beach, then a stay at an Airbnb in Rosarito, Mexico, according to social media posts from friends and family. But they never checked in to the Airbnb and Callum Robinson did not show up to work in San Diego as scheduled.

The missing men’s tents and burned-out truck were found on Thursday, by a remote stretch of coastline.

The brothers’ parents, Martin and Debra Robinson, told Australian news outlets they were heading to Mexico to be close during the search.

“Callum and Jake are beautiful human beings. We love them so much and this breaks our heart,” they said in a statement.

On Friday, four bodies were found in a covered-up well on an isolated ranch land six or seven kilometres from where the missing men’s car was found.

Three of the bodies had been there five to seven days before they were found on Friday, the authorities said. A fourth body was also found in the well, which was estimated to have been there 15 to 30 days.

Andrade Ramírez said authorities did not believe the attackers knew the victims were tourists, and emphasised that Baja California was still safe for tourists.

In 2023 there were more than 30,000 homicides in Mexico for the sixth consecutive year. More than 100,000 people are missing.

In 2015, Western Australian surfers Adam Coleman and Dean Lucas were murdered, believed to have been shot by gang members in the neighbouring Sinaloa region before their van and bodies were burnt.

Associated Press contributed to this report