Carter was 18 when he was arrested for a Facebook comment he made during a video-game argument. His bond was set at $500,000, which his family could not afford.
Carter will appear in Comal County District Court on July 16 for a bond hearing, in which his attorney will argue to have the bond reduced.
Attorney Don Flanary is representing Carter pro bono. The two met last Thursday, and Flanary learned that Carter has been placed on suicide watch.
"Justin is in bad shape and has suffered quite a bit of abuse while in jail," Flanary said. "We will likely bring out these issues at the bond hearing."
His father, Jack Carter, said he has had "concussions, black eyes [and been] moved four times from the base for his own protection."
"He's been put in solitary confinement, nude, for days on end because he's depressed. All of this is extremely traumatic to this kid," his father said. "This is a horrible experience."
Carter and a friend were on Facebook talking about "League of Legends," when they started arguing with a fellow player they knew only through the game. That player called Carter crazy, so Carter responded with a mock-threat.
"I'm [expletive] in the head alright. I think I'ma shoot up a kindergarten and watch the blood of the innocent rain down and eat the beating heart of one of them," Carter said. He followed up with an "LOL" and "J/K," letting him know he was kidding.
A Canadian woman saw the comment on Facebook, found where he lived, and reported it to his local authorities. The comment was posted just two months after the shooting at Sandy Hook Elementary School.
Carter was arrested February 21 and charged with making terroristic threats -- a felony that in Texas carries at least eight years in prison.
Notable deaths of 2014 [PHOTOS]
Soviets call Reagan joke 'dangerous' [ARCHIVE]