The eldest of the victims, Callum Robinson was a member of Australia's national lacrosse team and a resident of San Diego, just across the US-Mexico border from Baja California.

He graduated from Stevenson University in Maryland in 2015.

Callum was also diabetic, noted his mother, Debra Robinson, while appealing on Facebook for help to find her missing sons.

His girlfriend, Emily Horwath, posted several images of the couple on Instagram after his death was announced.

"My heart is shattered into a million pieces," she wrote. "I don't have the words right now."

In a statement posted online, Stevenson University athletics director Brett Adams wrote that Callum was "a stellar student, a stellar athlete and an even better friend".

"We are all so thankful that he was a part of our lives," his former coach, Paul Cantabene, said in the statement. "He lived an extraordinary life, but what is most impressive about Callum is what a loyal friend he was. Once you were his friend, you were friends for life."

A friend of Callum, Hayley Jacobs, told Australia's Nine News that he "lit up a room", adding that he eventually hoped to return to Australia.

"I asked him what his goals were in life and he just said that he wanted to go back to Australia and raise a family and grow," she added.

Ms Jacobs said she grew concerned after not hearing from Callum once his Mexico trip began. She saw him the day he left.

"I don't know what's going on but I think he's mad at me," she remembered thinking. "Like something doesn't seem right."

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