Boxer John VanMeter was shot and killed in his home Wednesday night.
Uvalde police say VanMeter's girlfriend called 911 around 8 p.m. to report an armed robber shot her boyfriend.
When police arrived to the house, they found VanMeter lying in the living room with a gunshot wound to the head.
He was transported to a hospital, where he was pronounced dead.
The 12-year-old is being held at a juvenile detention center on capital murder charges.
The boy's father told WOAI-TV he was "shocked and saddened" after learning about the murder.
"I wish this wouldn't have happened," said the boy's father. "I had a gut feeling like something was bothering me that day. Maybe all this could've been avoided and things would've been different."
According to the father, VanMeter and another man allegedly threatened his son with physical violence in October. He said he reported it to police.
When asked about the gun, the man says he has no idea how his son got it.
"There's no telling where it may have came from. Somebody from school or somebody from around here. Maybe he saw it and grabbed it somewhere," he told WOAI-TV.
According to ABC News, police did not say how they connected the 12-year-old to the shooting, but witnesses said they saw a male wearing a black bandanna over his face running from the home. Police have not revealed a possible motive in the shooting.
The boxing community in Uvalde is remembering VanMeter.
Tree City Boxing Club posted about him on Facebook, saying, "It's a sad sad night, not just for Tree City Boxing, but for our entire community. Please keep the VanMeters in your prayers."
Clutch City Boxing also posted about him on Facebook, writing "R.I.P. to the young champ John Vanmeter, gone way too soon."