A crossing guard was absent from a Harlem intersection where a 6-year-old boy was struck and killed by a tractor trailer on his way to school Thursday morning, a law-enforcement official said.

Police said Amar Diarrassouba was hit by the truck at around 8 a.m. while crossing at the corner of First Avenue and East 117th Street, about one block from Harlem's Public School 155. The boy was crossing First Avenue inside the crosswalk with his 9-year-old brother as the truck turned onto the avenue.

The driver of the tractor trailer continued driving about one block, apparently unaware he had struck the child, before he was flagged down by a witness, the official said.

The boy and his brother were taken by ambulance to Harlem Hospital where Amar died, the official said. His brother was found to be uninjured.

School crossing guards, who are part-time employees of the New York Police Department, are allowed to leave their posts for bathroom breaks and other circumstances. The law-enforcement official said it wasn't immediately clear why the crossing guard, whose identity hasn't been released, was away from the corner at the time of the accident.