SEOUL, South Korea — Ten people were killed and nine others were injured in South Korea after their bus caught fire on a highway, officials said Friday.

Twenty people, including the driver, were on the bus when it smashed into the guardrail and caught fire on the Gyeongbu Expressway near the southeastern city of Ulsan on Thursday night, the Ministry of Public Safety and Security said in a release.

Ulsan officials said the passengers are retirees of a petrochemical company and their spouses who were returning home from a tour to China.

The exact cause of the fire was under investigation, but authorities suspect it started after a ruptured tire caused the bus to hit the guardrail, with the friction from the bus sliding sparking flames, according to Ulsan officials.

Ulsan officials said the injured were treated with smoke inhalation and burns.