A ferry crew found 25 refugees inside a refrigerator container on a boat sailing from the Netherlands to Britain on Tuesday, emergency services said, according to AFP. The cargo vessel, bound for Felixstove, returned to the Dutch port of Vlaardingen, near Rotterdam.

“On board a ship it turned out that several people were found in a cooling container,” the Rotterdam emergency services said. “The ship returned to the harbour. The 25 people were taken off the ship and given the medical care they need. Earlier the message came from the ship that no people died.”

The service said that two people had been taken to hospital for a medical check-up. “Two people were transported to hospital for extra medical care. Twenty-three people were transferred to a police location after a medical check-up.”

The operator of the cargo ferry, DFDS Seaways, said that the refugees had come aboard a lorry trailer. The driver of the lorry trailer has been arrested.

The national origin of the refugees, who are mostly male, is not yet known, The Guardian reported. “The highest priority is to check how they’re doing medically…This is about human lives and the main thing is that they’re all okay,” Mirjam Boers, a spokesperson for Rotterdam police, told the daily.

A DFDS spokesperson said the container in which the refugees were found was meant to be sealed, but they had made a hole in it. He claimed that the security of the containers was “very tight” but added that this would be reviewed.

“We have surveillance and regular checks of the trailers,” he said. “It’s a mystery to us. We have good cooperation with authorities at all of our ports, but we have to realise that sometimes migrants do succeed in getting through.”

The incident follows the discovery of 39 bodies in a lorry in Essex, United Kingdom, last month. The lorry driver, a 25-year-old man from Northern Ireland, was arrested on suspicion of murder. No survivors were found in the vehicle, which entered the Welsh town of Holyhead in the Isle of Anglesey from Bulgaria. All the 39 people were Vietnamese.