Lucian Adrian Johnston, 20, was arrested Monday after deputies say he stabbed his two great-grandparents from the backseat of a car.
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He is facing two counts of aggravated assault with a deadly weapon, according to Sheriff Ed Gonzalez, and is now also charged with possession of a prohibited weapon in a correctional facility.
The sheriff's office said the Office of Inspector General is conducting an internal investigation.
Gonzalez said Johnston, his great-grandparents, who are 76 and 92, and his 12-year-old half-brother were in a car when Johnston got into a verbal argument with the couple.
According to the sheriff's office, the argument began after the great-grandparents told Johnston he could no longer live with them due to concerns about his recent behavior. Deputies said Johnston, from the backseat, began stabbing them in the head and neck with a pocket knife.
The 12-year-old boy is being hailed a hero after he wrestled the knife away from Johnston to save his great-grandparents.
"What I did was I grabbed him from behind. I pulled him backwards trying to get him away from grandma," said Sacha, who did not want to show his face during the interview.
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READ MORE: 12-year-old wrestled knife away from brother during stabbing: Sheriff
Johnston was set to appear in court overnight, but did not because he was undergoing mental health evaluations at the jail. Johnston's bond was set at $80,000, but was raised to $100,000 on Tuesday.
Johnston was also charged with assaulting a family member in 2017. The judge ordered he not be allowed to go anywhere near the great-grandparents.
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The video above is from a previous story.