Pharr Police Chief Andy Harvey told reporters the brothers admitted there had been a fight after they discovered Quintanilla had sexually assaulted his own biological daughter.
"What they learned was that their stepdad had inappropriately touched his 9-year-old, which is their half-sister," he said.
When he was killed, Quintanilla had an arrest warrant from 2019 for continuous sexual assault of another, unrelated minor, from 2014 to 2016. Police had been unable to find and arrest him since then.
"He was being elusive obviously, and hiding from us. They did not catch him, we did not find him," Harvey said.
He added police were unaware of the sexual assault of the nine-year-old girl, which allegedly happened the night before Quintanilla was killed.
According to investigators, the brothers confronted their stepdad at his trailer park home, and a fight broke out, during which Quintanilla left the residence. Alejandro followed and another fight broke out; soon afterward, Christian and Juan Eduardo caught up to them, and "it becomes a three against one," Harvey said.
They left him "beaten up, but still conscious and walking," and left in a car; but the trio switched vehicles to a pickup truck, and came back looking for him again, the Captain said.
A third fight broke out, and this time Christian was armed with brass knuckles, Harvey said. "This time, they severely beat him, to the point where he is either unconscious, or close to it."
He said the trio loaded Quintanilla onto the back of the truck, and dumped him in a field; he was unconscious and breathing when police found him, but he died at the scene.
All three teens are now charged with aggravated assault, and engaging in organized criminal activity, while Christian and Juan Eduardo are also charged with capital murder, as investigators believe they delivered the fatal blows.
Chief Harvey said the charge was capital murder rather than just murder, because Quintanilla was technically kidnapped when he was taken in the truck, and the suspects had also stolen jewelry from him before leaving him to die.
All three are being held on a $1million bond.
As of Friday morning, a Change.org petition to Texas Governor Greg Abbott asking all three be released had already surpassed 250k signatures.
"If convicted of capital murder, they face a minimum sentence of life imprisonment without parole," it reads. "We ask that the state of Texas to release these teenagers, who could possibly spend the rest of their life in prison for protecting their sister."