Careless driving may have been behind a tour bus fire late Thursday night that killed 10 passengers in the southern city of UIsan, police said Friday.

At least 10 people were killed and nine others were seriously injured after their bus slammed into a roadside guardrail and caught fire on an expressway near Ulsan late Thursday.

The deceased were trapped inside with smoke, as the front door was blocked by the guardrail, while those who survived managed to escape by breaking a window, police said.

Investigators suspect the bus driver’s abrupt attempt to change lanes on the expressway may have been the cause of the fatal accident.

The police arrested the 47-year-old bus driver, identified by his surname Lee, on charges of professional negligence and violation of the Traffic Accident Act. An application for a formal warrant to take him into custody is under review.

Allegations over the driver being sleep-deprived or drunk driving were confirmed to be untrue, the police said. But he had previously been convicted of 12 accounts of drunk driving.

According to CCTV footage, the tour bus, which was carrying 20 passengers, changed lanes to get ahead of a car and one of its tires exploded at 10:11 p.m. on Thursday. The bus crashed into the concrete guardrail and caught fire on the Gyeongbu Expressway near Ulsan, about 300 kilometers southeast of Seoul.

“After loud explosions, fire started to spread and the lights went off, with the bus filling with screams,” one of the survivors being treated at a nearby hospital was quoted as saying. “Without being able to find an emergency hammer, passengers tried to break the windows with their hands and feet to get out of the bus.

Most of the passengers -- retirees and their spouses from Hanwha Chemical -- were on the way home from the airport in Daegu after a trip to China.

Some critics have said lives could have been saved if there were fire exits on the bus.

The current law stipulates that buses and vehicles carrying more than 16 passengers are required to install fire exits, but it regards tempered glass to be a substitute for fire exits. 

By Ock Hyun-ju/(laeticia.ock@heraldcorp.com)