Police in Uvalde say John Duane VanMeter was 24 years old.
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VanMeter was also set to marry in a few weeks. His fiancee now says the boy had been making threats, KSAT-TV reports.
Sammy Arellano, VanMeter's fiancee, said the 12-year-old boy was the best friend of her son at one time.
"His family is not speaking to me. They blame me for it," Sammy said.
She and her mother say his family won't allow her to be a part of the funeral.
"We want to be part of it, but they're not letting us be part of it. We're just heartbroken," Norma Arellano said.
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VanMeter's fiancee says the couple had agreed to take him in their home.
"He didn't mind, he didn't care, as long as they stayed out of trouble," Sammy said.
She says she brought the 12-year-old and her son to take boxing lessons at the same gym where VanMeter would train.
"They just kept getting in trouble. I told him he wasn't allowed at my house anymore and this was months ago," Sammy said.
According to Sammy's mother, the boy kept threatening to kill her 10-year-old grandson.
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"Three months later he comes back and he really did it, but to the wrong person," Norma said.
The two women say they will remember VanMeter as being a great role model to young boxers.
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