Missing surfers/Mexico
Rescuers are seen working at a clifftop shaft where bodies believed to be those of three missing surfers were discovered. AFP

Three bodies believed to be those of two Australian brothers and an American who disappeared on a surfing trip in Mexico have bullet wounds to the head, authorities said Sunday.

Relatives of Australians Callum and Jake Robinson, and their American friend Jack Carter, were in Mexico to aid in the identification process, state prosecutor Maria Elena Andrade said at a press conference.

"The relatives arrived in the United States (on Saturday) and today they presented themselves to the prosecutor's office," she said.

The victims "all have a hole in their head made by a firearm projectile," Andrade added.

The motive was believed to the attempted theft of their pick-up truck, she said.

The vehicle -- which had been burned -- was found nearby.

Three suspects, one of them a woman, have been detained on suspicion of direct or indirect involvement in this case, according to prosecutors.

Investigators said earlier that the bodies, which were recovered from a cliff-top shaft in the crime-hit northwestern state of Baja California, were very likely those of the missing tourists.

The bodies were in an "advanced state of decomposition," Andrade said.

But "given their clothing and certain characteristics such as long hair and specific physical descriptions, we have high probability," that the bodies are those of the three missing men, she said.

Another body found at the site had been there longer and was unconnected to the others, officials said.

AFP journalists saw the authorities use a pulley system to extract the mud-covered bodies from the shaft on Friday near the town of Santo Thomas, about 30 miles (45 kilometers) southeast of Ensenada.

The Australian brothers' mother, Debra Robinson, had sounded the alarm on a Facebook page for Baja California tourists several days ago, after the young men dropped out of contact.

A missing poster shared on social media said Callum Robinson was 33 and his brother Jake, 30. It named their friend as Jack Carter Rhoad, aged 30.

Callum Robinson's Instagram page showed several images from the trio's Mexico trip: enjoying beers with their feet up in a bar, lazing in a jacuzzi, eating roadside tacos, looking out at the surf.

Six-foot-four (1.93 meters) Callum had played in the US Premier Lacrosse League, which left a message on its website saying the lacrosse world was "heartbroken by the tragic loss" of the trio.

Jake Robinson was a doctor in Perth, according to Australian media.

Baja California is known for its inviting beaches, and its resorts are popular with US tourists, partly because of their proximity to the border.

It is also one of Mexico's most violent states because of organized crime gangs, although cartel activity doesn't commonly affect foreign tourists.

The latest case echoes that of two Australian surfers who were murdered and their bodies burned while traveling in the northwestern Mexican state of Sinaloa in November 2015.

Criminal violence in Mexico has claimed 450,000 lives and led to more than 100,000 disappearances since the end of 2006, when the government launched a controversial anti-drug strategy involving the military.