The boy was to be charged with criminally negligent homicide, Officer Doris Garcia said. Police were withholding his name because of his age, she said.
Amber Sadiq, 8, was holding hands with her brother, crossing the street near her school Monday afternoon when she was struck by the bus in the Brooklyn neighborhood of Crown Heights. Police believe the boy was alone on the bus, Garcia said.
The New York Daily News reports that Amber's 10-year-old brother, Umar, had just picked her up from school, and barely managed to get out of the bus' path.
"I was right there with her. It was terrible," Umar, told the newspaper, sobbing. "Someone picked me up and carried me home."
"She tried to run but the garbage can was in the way," said 12-year-old Kassandra Polanco, who saw the bus bearing down on the girl.
Her mom, Reina Herrera, ran from her apartment screaming, "My baby! My baby!" as she tried desperately to lift the bus off her daughter, Amber's aunt told the New York Post.
Investigators believe the driver got off the bus and secured it but the boy may have gotten inside through the back door, an emergency exit that cannot be locked, police spokeswoman Detective Theresa Farello said. No charges were filed against the driver.
A police source told the Daily News that officials were looking for a second boy believed to be involved.
A woman who saw what was happening jumped into the bus, but was unable to stop it in time, police said.
Deli worker Sam Ahmad said he saw the girl under the bus and called emergency services. He said he and about a dozen other men tried to lift the yellow bus to free her but it was too heavy.
Police said the girl was 8; the Department of Education said she was 7. Amber's family remembered her as a bright girl who lit up every room she walked into, the Daily News reports.
"She was sunshine," Lucy Caba told the newspaper. "She was such a happy kid. No matter what, she always had a smile."