The Tochigi Prefectural Police on Sunday raided an office of a Tobu Railway group company on suspicion of professional negligence resulting in death in connection with a fatal accident in which a limited-express train struck four landscapers at a station on the Tobu Nikko Line last week.

According to Tobu Railway, the weed-clearing work had been contracted out to Tobu Kensetsu, based in Nikko, Tochigi Prefecture. Although workers were still on the railway tracks, the train driver said he had recognized a signal indicating that the workers had left the area. Police are investigating the circumstances and cause of the accident.

The accident occurred around 10:45 a.m. Thursday at Shin-Kanuma Station. Four male workers in their 50s and 60s who were clearing weeds on the tracks were killed after being hit by the six-car Spacia X No. 2 limited-express train bound for Tokyo’s Asakusa Station from Tobu-Nikko Station in Tochigi Prefecture.

According to the railway operator, there are internal regulations governing worker evacuation procedures.

When a train approaches, lookouts signal other workers with whistles and other devices. Workers then stop their work and evacuate from the tracks while communicating with lookouts.

After confirming that all workers have moved to a safe location, a lookout signals the train driver by holding a yellow flag horizontally. The driver is required to sound the horn upon seeing the flag.

At the time of the accident, three lookouts were assigned along the tracks, including one positioned about 80 meters from the work site who was responsible for raising the flag.

A relay lookout was also assigned to signal when the train came within about 500 meters of the work site. However, the relay lookout reportedly said that it was “not possible to communicate with the train lookout.”

The driver of the express train has said that he saw the yellow flag from the train lookout and sounded the train’s horn.

The Tochigi police established a special investigation squad at Kanuma Police Station on Friday and began investigating the case as one involving professional negligence resulting in death.