A woman arrested over the deaths of brothers Jake and Callum Robinson has turned on her ex-partner, claiming he confessed to killing the “gringos”.

The woman, who is in custody along with two men, said Jesus Gerardo (alias El Kekas) told her he killed the Robinsons and their American friend Carter Rhoad.

The shocking allegations were heard in a tiny courtroom in Ensenada, Baja California, on Thursday (AEST) as prosecutors laid out some details of their case.

The bodies of Jake, 30, Callum, 33, and Rhoad, 30, were found down a 15-metre well after they went missing on a surfing trip on a remote coastline.

They each had a single gunshot wound to the head, with police theorising they were killed in a robbery gone wrong.

Prosecutors alleged that after the men were murdered, El Kekas told his then girlfriend Ari Garcia: “I f–cked up three gringos”.

When she asked what he meant by that statement, the Mexican responded: “I killed them.”

Prosecutors say El Kekas then gave Garcia a mobile phone that belonged to one of the victims.

He also put tyres on the woman’s car that investigators allege were  stolen from the Chevrolet ute belonging to the travelling surfers.

Police arrested Jesús Gerardo Garcia Cota (El Kekas), his ex-partner Ari Gisel García Cota and his brother Christian Alejandro Garcia last week.

Charges against El Kekas have not been upgraded to murder.

On Saturday, a criminal charge of forced disappearance, equivalent to kidnapping, was laid against him.

El Kekas appeared in court in an orange jumpsuit, with cuffs around his ankles that were chained to the floor.

He has a criminal record of drug dealing, vehicle theft and domestic violence.

Sunrise reports the woman, who has a three-year-old child, told investigators she wanted to cooperate because she did not want to be involved.

She was the first person to be arrested over the men’s disappearance when she was found in possession of one of their phones during a major search for the men.

Perth siblings Callum and Jake Robinson, both in their 30s, went missing on a surfing and camping trip.

Meanwhile, Australian Federal Police are working with Mexican authorities to bring justice for the two murdered brothers.

Foreign Minister Penny Wong said Australia’s embassy in Mexico would continue to work with federal police and local authorities.

“I am heartbroken for Jake and Callum’s parents and am thinking of them at this incredibly difficult time,” she said.

Parents of the slain Australians, Debra and Martin Robinson, said the world had since become a darker place and were preparing to bring home the bodies of their sons.

Mexican prosecutors believed more people than the three detained suspects have been involved.

The AFP has a presence in Mexico and is liaising with local authorities.

-with AAP