PHOTOS: Verdict in Howard High death
There was no sufficient evidence showing that the girl found not delinquent had participated in the plan, Coonin said.
Joyner-Francis was 16 when she died April 21, 2016, after the violent encounter with Carr at Howard High School of Technology. Dozens of girls watched the confrontation as it unfolded in the school's second-floor bathroom, and at least two recorded segments of it on their cellphones.
During trial proceedings this month, the prosecution played the video clips, which showed Carr punching and kicking a prone Joyner-Francis after she fell to the floor. After onlookers pulled Carr away, Joyner-Francis could be seen clinging to what appeared to be Carr's shirt.
Joyner-Francis died a short time later after her breathing became labored and she lost consciousness while lying on the floor in the handicap bathroom stall, according to witnesses.
The state medical examiner, Dr. Adrienne Sekula-Perlman, testified that Joyner-Francis suffered sudden cardiac death brought on by a rare combination of heart and lung defects. Contributing factors included the emotional and physical stress of the fight, which triggered a fight-or-flight response, she said.
Joyner-Francis' autopsy revealed she had a hole in the atrium section of her heart and stiffened, narrow arteries in her lungs. She also suffered soft tissue injuries from the fight, Sekula-Perlman said. She called the case a homicide and said the teen "would probably still be alive" if not for the stress of the confrontation.
A witness for the defense, Dr. Richard Ringel, a professor of pediatrics at Johns Hopkins School of Medicine, said the severity of Joyner-Francis' rare condition made it impossible to conclude whether Joyner-Francis probably would be alive today.
“It might have been the next day or sometime down the road,” he said.
Coonin disagreed, stating the evidence shows "beyond a reasonable doubt that the death of Amy Joyner-Francis was caused by the action of Trinity Carr."
"While it may be true that Amy Joyner-Francis, due to her condition, could have died from a multitude of stressors, until such an event occurred, if it were to have occurred at all, she had a right to live one more day, one more week, one more month or year until her time, without a contributing cause by another," Coonin said.
During closing arguments Tuesday, an attorney for the prosecution insisted the confrontation was an attack, "not a fight."
Defense attorneys said the confrontation, while tragic, was between mutual combatants and no one, even doctors, could have predicted the deadly result.
All three girls decided not to testify.
USA TODAY is not naming two of the teens because one was acquitted and one faced less-serious charges.
Coonin ruled last year that Carr would be tried as a juvenile. Had she been tried and convicted as an adult, she would have faced up to eight years in prison. Being declared delinquent, she would be subject to supervision until age 19.
Coonin said evidence showed that Joyner-Francis' death was the result of an attack with the intent to harm, not a fight. He added that Carr's failure to perceive that risk of such an attack constituted a gross deviation from the standard of conduct of a reasonable person.
"The attack carried out by Trinity Carr in the close confines of the school bathroom stall posed risk of potential catastrophic physical harm including death by virtue of the tile floor, walls and fixtures," Coonin said. "Had a death resulted from internal bleeding after striking her head on the floor would that result in any way change the risk that the assault itself created?"
During proceedings attorneys litigated questions such as what a “reasonable” 16-year-old should expect as a result from a fight, what type of clothing is worn by kids preparing to fight, and whether Joyner-Francis was a victim of a planned attack by the three girls, as prosecutors allege.
Contributing: The Associated Press. Follow Karl Baker on Twitter: @kbaker6