Police are investigating the mysterious death of a cleaner following an early morning shift on premises belonging to Dutch rail operator NS in Utrecht earlier this week.

According to witnesses, the man and his colleague fell ill after they had finished cleaning the trains. Their colleagues then alerted emergency services and the men were taken to hospital. One man died the next day while the other is still in hospital.

NS were told what happened on Thursday, a spokeswoman confirmed to the Telegraaf. The news came as “a big shock” NS said. The rail operator confirmed that the men lost consciousness but said the cause was unclear. “We know the yard is safe. But it is now up to police to investigate further,” the spokeswoman said.

A spokesperson for cleaning company Vegebo, which employed the men, told the paper, they had no idea what might have happened but said the company had offered its condolences to the man’s friends and family.