The child went missing in St Johns County, Florida, on Sunday and was found dead hours later.
She died of “force trauma by stabbing” and her death has since been ruled a homicide by the medical examiner, the St Johns County Sheriff’s Office announced on Tuesday.
“This is a cold-blooded murder of a 13-year-old young girl who did not deserve to die,” St Johns County Sheriff Rob Hardwick told local news outlet WJXT.
“That being said, I don’t wanna mention the number of times [Tristyn was stabbed], but it is horrific. It is horrible,” Mr Hardwick added. “And the word accident is nowhere involved in this case.”
A 14-year-old boy, identified as Aiden Fucci, was arrested this week in connection with Tristyn’s death and charged with second-degree murder. The suspect appeared in court for the first time on Tuesday.
He is currently being held by the Department of Juvenile Justice, as the 7th Circuit State Attorney’s Office has yet to determine if they will try him as an adult.
“That’s actually a decision we made collectively ... with the State Attorney’s Office,” Mr Hardwick said on the charge. “We can always increase the charge.”
Both Tristyn and the suspect attended Patriot Oaks Academy, a public K-8 school, and grew up in the same neighbourhood, Mr Hardwick said.
The last time Tristyn was seen alive was a little after midnight on Sunday in her Durbin Crossing neighbourhood. Her body was then found around 6pm local time by a local resident after authorities put out a missing person alert.
Mr Hardwick said his office has yet to determine a clear motive for the murder.
”We’re going to look at every aspect, every crime that’s out there. Our guys and girls are going to leave no stone unturned,” Mr Hardwick said. “So those facts will come forward as the office of the medical examiner will actually complete their investigation.”
During the Tuesday morning hearing, a judge ordered the suspect to be held in juvenile custody for at least 21 days. The suspect has not yet entered a plea for his second-degree murder charge.