MIAMI (AP) — A 13-year-old girl was shot to death while riding a charter school bus with her younger sister today and a male student was being held for questioning, Miami-Dade Police said.

The male student was in custody but authorities did not release his name or age and said they are still interviewing him. A gun was also recovered at the scene in Homestead, south of Miami, but authorities didn't say where or how many times the victim was shot.

"We still don't know what the motive may have been," said police spokesman Det. Alvaro Zabaleta.

Eight other children, including the victim's 7-year-old sister, were on the bus but were not harmed. Authorities took the children and the bus driver to a police station to be interviewed.

The victim, who has not been identified, attended Palm Glades Preparatory Academy, a charter middle school south of Miami. The victim's sister went to nearby Summerville Advantage Academy.

Concerned parents gathered at the scene, which was cordoned off by crime scene tape as authorities combed the bus for evidence and gathered children's backpacks and belongings.

Grief counselors were on their way to the charter school, said Lynn Norman-Teck, a spokeswoman for Florida Consortium of Public Charter School.

The school principal walked through every classroom Tuesday morning to make sure the students were safe.

A phone message left for the private school bus company was not immediately returned.