A city bus carrying three passengers crashed into nearly a dozen vehicles in Seoul on Wednesday, killing the driver and a commuter, police said Thursday.

The No. 3318 bus was rolling through the Seokchon Lake junction at about 11:50 p.m. when it veered into a row of taxis and drove 1.2 kilometers further off-course, strking at least six more vehicles, according to the Songpa Police Station. Seventeen people affected by the accident sustained serious injuries.

Police said a black box from the bus was damaged after the point of the first crash, making identification of the cause of the accident more difficult. Investigators are now reviewing a number of possible scenarios, including that the driver was drunk and mechanical failure.

“The driver did not take any preventive measures before the crash, such as turning the steering wheel to the side. We’re checking the driver’s health records for possibility of his stroke or heart attack,” a police official said.

The bus operator said the GPS transmitter was deliberately turned off a few stops before the accident occurred, adding more uncertainty about whether the accident was intentional.

The driver, identified by his last name Yeom, crashed into three taxis at the junction and accelerated the bus toward the Songpa District Office, where he struck five cars in a row.

Witnesses reported hearing an explosion when the bus finally came to a stop after crashing into a passenger bus, which was carrying at least seven passengers. A 19-year-old passenger surnamed Lee, who was sitting at the back seat of the second bus, was killed.

Survivors on the No. 3318 told reporters they shouted at Yeom to stop after the first collision, but he kept driving.

The bus operator denied allegations about Yeom’s health problems, saying that he was “healthy enough to run a full course marathon last weekend” and that “no abnormal findings were seen in his medical examination last year.”

The operator also played down the possibility of a mechanical failure on the bus, a 2013 model, claiming it passed a regular inspection the day before the accident.

Police have secured black box recordings from four vehicles hit by the bus to find the exact cause of the accident.

Officials said they would send the driver’s body and the damaged black box to the National Forensic Service to find out what happened during the crash.

By Suk Gee-hyun (monicasuk@heraldcorp.com)