Police on Tuesday raided more Seoul Metro offices over a subway collision that injured more than 240 people last week.

Investigators seized documents and data on traffic signals from the mechanics department at its headquarters in Gangnam and annexes in Euljiro and Dongdaemun stations, police said.

They also confiscated documents from a private firm in charge of entering signal data in the system, alleging that the accident was caused by a signal failure.

“The operator changed signal data on April 29 and noticed errors on the signal monitor on May 2. But an official in charge ignored the problem, thinking that it was just a common error,” Baek Gyeong-heum, senior investigator at Seongdong District Police Office, said in its interim report.

On Friday, a train rear-ended another train that had stopped at Sangwangsimni Station on Line 2 at around 3:30 p.m., leaving at least 246 passengers with mild injuries such as cuts and bruises.

City officials said that the malfunctioning of a traffic signal system is thought to have been the cause of the accident.

According to Seoul Metro, the automatic train stop system at the station, which is designed to maintain safe distance between trains, didn’t activate properly at the time, sending the wrong signals to the incoming train.

The conductor of the approaching train applied the emergency brake when he saw the train ahead, reducing the speed from 68 kilometers per hour to 15 kilometers per hour before the collision, city officials said.

After being closed for nine hours, a section of Line 2 ― between Euljiro 1-ga Station and Seongsu Station ― was fully resumed.

Police on Saturday raided Seoul Metro’s headquarters in Gangnam, focusing on the cause of the accident and analyzing communication logs.

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