BRUSSELS (Reuters) - Belgian police found 12 migrants alive in a refrigerated truck at a motorway parking area in northern Belgium on Wednesday after the driver alerted the authorities, a federal police spokeswoman said.

The driver, who was transporting fruit and vegetables, called police after he found 12 adult men - 11 Syrians and one of Sudanese origin - in the truck on Wednesday morning after a stop for the night. Police took them to the Immigration Office in the city of Antwerp. All were in good health, police said.

A spokeswoman for the federal police said the prosecutor’s office in Antwerp had opened an investigation into the incident, which occurred on motorway E34 near the town of Turnhout.

The investigation is to determine the planned route of the truck, how the migrants climbed on board and if they were trying to reach Britain.

Last week, 39 migrants were found dead in a refrigerated truck in Britain after crossing from the Belgian port of Zeebrugge. Belgian authorities, facing increasing scrutiny of their fight against human trafficking, are cooperating with a British investigation.

Migrants sometimes choose refrigerated trucks to cross borders because their construction makes it more difficult for dogs and heat scanners at checkpoints to detect stowaways.

Belgian paper De Morgen reported that in the Turnhout incident, the truck driver and his co-driver were Polish and called the police after noticing on Wednesday morning that the seal on the trailer was broken.