The Facebook dispute between the two girls over the boy ended when one gunned down the other on a Southside Chicago porch, police said.
Endia Martin was fatally shot in the after-school confrontation outside her home in the Back of the Yards neighbourhood, the Chicago Tribune reported.
An honours student charged in Endia's murder appeared in juvenile court yesterday.
"Bout to beat some a--" the girl tweeted about two hours before the fatal confrontation on Monday.
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Endia was with friends on the front porch when the suspect and her group approached, Cook County prosecutors said.
The suspect took out the gun and pulled the trigger, but the gun "malfunctioned", said Assistant State's Attorney Kathleen Kain.
The girl handed the gun to others in her group who quickly fixed it. The girl raised the gun again and opened fire, killing Endia and striking a 16-year-old girl in the upper left arm who was later released from a hospital.
After Kain called the suspect "a danger to the public and herself," Juvenile Court Judge Stuart Paul Katz ordered her held in custody.
Superintendent Garry McCarthy hinted that a 24-year-old uncle of the suspect's also was being questioned for giving the gun to the girl, knowing "there was going to be a fight", he said.
McCarthy said the gun used to kill Endia had been stolen from a car on April 14.
"What would have been, under any other circumstance, probably a fistfight between 14-year-old girls because they were fighting over a boy turned into a murder," McCarthy told reporters.