Two Australians and an American were doing what they loved on the stunning, largely isolated stretch of Baja California's Pacific coast. Their last images on social media showed them sitting and gazing at the waves.

What happened to end their lives may have been as random as a passing pickup truck full of people with ill intent. The surfers were shot in the head, their bodies dumped in a covered well miles away. Here's what we know:

Who were the victims?

Brothers Jake and Callum Robinson from Australia and American Jack Carter Rhoad had apparently stopped to surf the breaks between Punta San José, about 50 miles south of Ensenada, and La Bocana, further north on the coast.

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.

Callum, who was six-foot-four, had played in the U.S. Premier Lacrosse League, which left a message on its website saying the lacrosse world was "heartbroken by the tragic loss" of the trio.

"We offer our hearts, support and prayers to the Robinson and Rhoad Families, as well as all who loved Callum, Jake and Jack," it said.

On social media, his devastated girlfriend shared a black and white photo of the couple kissing, with the message: "You are one of one. I will love you forever."

In another photo she shared, in which the two can be seen embracing,  she wrote: "My heart is shattered into a million pieces. I don't have the words right now."

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

Rhoad was engaged and set to marry Natalie Weirtz on Aug. 17 in Columbus, Ohio, according to a Zola online wedding registry. A GoFundMe which was launched to "rally support for Natalie" had raised over $57,000 as of Tuesday morning.

"In the wake of the heartbreaking loss of Carter Rhoad, Callum Robinson, and Jake Robinson, our hearts ache with grief for their families, friends, and community," reads a separate GoFundMe set up for Rhoad's family.

What happened to them?

The three friends were attacked there on April 28 or 29.

As soon as police arrived at their last known camp site, it was clear that something had gone violently wrong.

There were bloodstains and marks "as if heavy objects had been dragged," leading to suspicions of an attack, the Baja California state prosecutor's office said in an attempt to reconstruct the scene.

Chief state prosecutor María Elena Andrade Ramírez described what likely would have been moments of terror that ended the trip for the three men.

She theorized the killers drove by and saw the foreigners' pickup truck and tents and wanted to steal the truck's tires and other parts. But "when (the foreigners) came up and caught them, surely, they resisted."

She said that's when the killers would have shot the men. She said they were likely not attacked because they were tourists. "The evidence suggests they (the killers) did not know where they were from."

Andrade Ramírez said the reconstruction of events was based on the forensic examiner's reports, noting all three had bullet wounds to the head.

There was a hurried attempt to destroy evidence. The foreigners' tents were apparently burned. The pickup truck was driven miles away and burned. The assailants' truck was later found with a gun inside.

Then, at "a site that is extremely hard to get to," the bodies were dumped into a well about 4 miles (6 kilometers) away. Investigators were surprised when, underneath the bodies of the three foreigners, a fourth body was found that had been there much longer.

"They had to have previous knowledge of it," Andrade Ramírez said of the attackers, acknowledging the possibility they were behind the previous killing.

The well had been covered with boards. "It was literally almost impossible to find it," Andrade Ramírez said. It took two hours to winch the bodies out.

Who are the suspects?

Prosecutors have said they were questioning three people in the killings. Two were caught with methamphetamines. One of them, a woman, had one of the victims' cellphones when she was caught. Prosecutors said the two were being held pending drug charges but continue to be suspects in the killings.

A third man was arrested on charges of a crime equivalent to kidnapping, but that was before the bodies were found. It was unclear if he might face more charges.

The third man was believed to have directly participated in the killings. In keeping with Mexican law, prosecutors identified him by his first name, Jesús Gerardo, alias "el Kekas," a slang word that means quesadillas, or cheese tortillas.

Andrade Ramírez said he had a criminal record that included drug dealing, vehicle theft and domestic violence, adding, "We are certain that more people were involved."

She emphasized that she could not discuss anything related to the suspects, or their possible statements, because that was not allowed under Mexican law and might prejudice the case against them.

Andrade Ramírez noted that the victims' families said the brothers and Rhoad had come many times to the seaside spot and never had any problem. This time, however, "there was no way to ask for help when the attackers showed up."

What's next?

In a statement to CBS News, the FBI said it "continues to work with our international law enforcement partners in resolving this ongoing situation" and has been in contact with the family of the American victim.

"While we cannot comment on specifics to preserve the sanctity of the investigative and legal processes, along with protecting the privacy of those impacted, we can assure you that we are assessing every tip. If credible, we will pursue those leads with rigor," the FBI said.

Australian Prime Minister Anthony Albanese said on Tuesday he had requested an opportunity to speak to the brothers' parents Deborah and Martin Robinson.

"This is a terrible tragedy and my heart goes out to them. To identify these wonderful young men and they have been travelling in Mexico. We've been dealing with them through the Department of Foreign Affairs and Trade. I've indicated that I wish to speak to them at an appropriate time of their choosing," Albanese told reporters in the Queensland state town of Rockhampton.

AFP contributed to this report.

In:
• Mexico
• Murder