A bus driver who caused a tunnel accident that killed four people last month was found to have been using a phone behind the wheel, which resulted in a delayed response to the changing traffic conditions, police authorities said Tuesday.

According to Boeun Police Station, the 59-year-old man, who had been receiving treatment for injuries at a hospital, admitted to checking text messages on his phone when the accident occurred.

On Oct. 21, a tourist bus rear-ended a 15-seat van inside Suriti Tunnel along the Dangjin-Yeongdeok Expressway in Boeun-gun, North Chungcheong Province, resulting in four fatalities and seven injuries.

The bus failed to decelerate and collided with the van caught in traffic inside the tunnel. The van’s all 11 passengers were friends from elementary school on a weekend trip and were in their 60s. All the fatalities were from this van.

Of the 26 passengers on the bus, one sustained a serious injury, while the driver had a minor injury, police officials said.

Police have handed over the bus' black box to the Korea Road Traffic Authority for examination. KoROAD will conduct a simulation test based on the black box footage to determine the speed of the bus at the time of the collision and whether the bus driver applied the brakes.

Black box footage from other vehicles in the tunnel showed that the bus did not slow down even before crashing with the van.

The police will decide whether to seek an arrest warrant for the bus driver, depending on the results of the analysis, officials said.