SEOUL, South Korea (AP) — A driver involved in a harrowing car accident in Seoul that killed nine people will face an investigation on alleged accidental homicide, police said Tuesday.

On Monday night, a passenger car hit pedestrians waiting at a crowded traffic light in central Seoul, after reportedly driving in the wrong direction and colliding with two other cars. The incident left six others injured, including the driver who was detained on the spot.

Police said Tuesday that tests showed the driver wasn’t under the influence of alcohol or drugs.

The driver told investigators that his car accelerated abruptly and unintentionally. Police said they’ll consider seeking a formal arrest warrant to extend his detaining if it’s needed for an investigation. Police said his car would be sent to the country’s forensic agency for an examination.

Police refused to disclose the identity of the driver, citing privacy-related regulations but South Korean media reported he is a bus driver in his late 60s from Ansan, a city just south of Seoul, who has 40 years of driving experience.