Exactly what caused the accident remains under investigation.

“The train saw them from a long way away and signaled, but they weren’t able to get out of the way,” police spokesperson Björ Blixter to the TT news agency.

Several of the victims’ classmates from nearby Nösnäsgymnasiet high school witnessed the accident, prompting the school to call a crisis meeting shortly after the accident, the Göteborg-Posten (GP) newspaper reported.

According to the newspaper, the accident took place at an unguarded crossing across the tracks about 500 metres from the school which featured signals and lights, but no booms, and was often used by students to cross the track on their way to and from school.

In addition to the students, there were several others who witness the event, many of whom will be interviewed by police in an effort to learn more about the exact chain of events.

“We consider what happened to be an accident, but we’re going to investigate what happened,” said police spokesperson Stefan Gustafsson.

Emergency services were called about the accident shortly after 1.30pm.

The train was traveling north on a single track just south of Stenungsund station at the time of the accident.

Two trains had to be rerouted and at least on train remain stuck on the tracks while authorities worked to clear the track at the scene of the accident.

“Buses have been called in to replaced the cancelled trains,” Lars Hedström of the Swedish Transport Administration (Trafikverket) told TT.

Trains along the line, known as the Bohusbanan, were taken out of service for several hours during the afternoon while crews examined signals along the stretch of track near the accident site.

“We’re going to test the signaling equipment before trains can be let through,” said Hedström.

By 5pm, trains were once again running normally, but with delays of up to 30 minutes.