WBA featherweight world champion Carl Frampton was among the many who paid tribute to his former opponent - Alejandro Gonzalez Jr, who was only 23, was found executed alongside two other people in a truck near his home in Guadalajara on Friday.
One of the other bodies is believed to be the young boxer's grandfather.
Frampton defeated the Mexican boxer last year after surviving two knockdowns in the first round in Texas.
The Northern Irishman tweeted: "Just hearing the tragic news about Alejandro Gonzalez Jr. One of the nicest guys I've shared a ring with. RIP Champ"
Frampton's wife also paid her respects, tweeting: "Absolutely heartbreaking news about the passing of Alejandro Gonzalez Jr. Spoke at length with him in El Paso about our families. So sad."
Gonzalez Jr had won 25 of his 31 fights but had lost one and drawn the other of his final two bouts.
Nicknamed Cobrita, he was the son of former featherweight world champion Alejandro Gonzalez. He caused Frampton plenty of early problems before the Belfast man found his rhythm.
After losing to Frampton, he was shocked in losing a very closer ten round decision to Karim Guefri, who survived a first round knockdown. He then fought to a twelve round majority draw with 9-14 journeyman Hector Bobadilla.
The performance against Frampton earned him a contract with manager/adviser Al Haymon, who then featured the boxer