Roma High School junior Robert Garza, 16, was playing in the AAU tournament on Saturday with the Hoopsters, a South Texas club team, when he collapsed without any warning.
"It's unexplainable. He got a glass of water. He 'high-fived' Pablo Adame (a Hoopster teammate) and then it just happened. He collapsed," Hoopster coach Arnold Martinez told The Monitor of McAllen.
The cause of death was not yet known, Martinez said.
"During pregame warm-ups, he was dunking the ball and laughing with his teammates. Everything looked good," he told the newspaper.
Paramedics administered CPR, but Garza was pronounced dead at Austin's Brackenridge Hospital just more than an hour later, said Abelardo Escobar, Garza's high school coach. An autopsy was planned.
"Robert was a great, great human being," Escobar told The Monitor. "I'm very, very sad. The Lord asked for an angel to come to Him today. He took care of his body. He ate all the right things. I don't know how else to explain it."
Calls by The Associated Press to Martinez and Escobar went unanswered on Sunday.
