Police plan to summon two officials of a Seoul subway operator Thursday for questioning over their handling of signal errors detected hours before a recent train collision that injured over 200 passengers.

The summonses are part of the ongoing investigations into what caused the accident in which a moving train rammed into the rear of another train that had stopped at Sangwangsimni Station due to mechanical problems on Friday afternoon. Some 238 people suffered minor injuries.

The investigation team at the Seoul Metropolitan Police Agency said it will question the two Seoul Metro officials in charge of controlling the automatic train stop system of Subway Line 2 over what measures they took upon learning that its automatic stoppage system did not properly work.

The ATS, designed to keep a safe distance between trains, is supposed to activate when trains are within 200 meters of each other, but it didn’t function at the time of the incident.

Early Friday, one of the two officials set to come under a police probe received an order from his fellow colleague to check what caused the signal errors, according to investigators.

“We will ask the official about whether he found the cause of the error and what actions he took afterwards,” a police officer said, adding he believes another official set to face questioning will have some knowledge about the situation as he was on the signal team on the day of the accident.

“We will also check closed-circuit television data and what relevant officials say to exactly learn what brought the signal system down,” he added.

Wednesday’s probe into two other employees found that one of them first detected a signal error 14 hours before the accident but didn’t take any corrective action.

An investigation showed that the signals first malfunctioned on April 29, when Seoul Metro updated its signaling system on Line 2.

But officials and drivers hadn’t been aware of the problems because trains hadn’t come close to one another at stations until the accident on Friday.

According to Seoul Metro’s labor union, the ATS system has frequently experienced problems since it was introduced in 2002 to reduce labor costs.

Subway Line 2 is the busiest route of the capital city with a population of about 10 million people. In 2012, 752 million passengers rode the line, the most among all others, according to government data.

The accident caused by human error was another blow to South Korea, which is still reeling from a ferry tragedy on April 16 that has left 304 people dead or missing. The 6,825-ton Sewol carrying 476 people sank off South Korea’s southwest coast, sending the nation into grief and shock. (Yonhap)