A 16-year-old boy was arrested Monday after he allegedly shot and critically wounded another 16-year-old in downtown Los Angeles, at about 3 p.m. on Third Street, just west of Hill Street, police said.

A police officer who was nearby witnessed what happened and took the alleged shooter into custody without incident, said Los Angeles police detective Gus Villanueva.

A motive for the shooting was unclear, he said.

Because the suspect is a juvenile, his name will not be released. Police also withheld the victim’s name.