Prosecutors raided the office of the operator of a subway line on Saturday to look into what caused a train collision that injured more than 200 passengers a day earlier.

Two trains collided on Seoul's busiest subway line at around 3:30 p.m. on Friday, injuring 238 passengers. No fatalities have been reported. Some 1,000 other passengers escaped safely around 10 minutes after the collision.

The collision happened at Sangwangsimni Station on Subway Line 2 when a train ran into the back of another train that had stopped at the station due to a mechanical problem.

Prosecutors plan to analyze documents and other data to find the cause of the accident.

Seoul city officials said a failure in the following train’s automatic distance control system (ATS) could have been the cause of the collision. The ATS should have activated when the following train was within 200 meters of the other train.

Operations for Line 2 have fully resumed, after a small section of the line was closed for nine hours.

In 2012, 752 million passengers rode Line 2, the most among all other lines, according to the latest data.

The accident comes as the country is still reeling from a ferry tragedy on April 16 that has left 302 people dead or missing. The 6,825-ton Sewol carrying 476 people sank off South Korea's

southwestern island of Jindo, sending the nation into grief and shock. A total of 174 people were rescued in the accident. (Yonhap)