London (CNN) All 39 people who were found dead in a truck in a UK industrial park on Wednesday were Chinese nationals, British police said Thursday, as officers in England, Northern Ireland and Belgium scrambled to establish the chain of events that led to the tragic incident.

The cause of death of the eight women and 31 men has yet to be established, Essex police said in a statement. The victims were discovered in a container at the back of the truck, at an industrial park in Grays, Essex, a town 20 miles east of London.

Police believe the container came from Belgium into England early Wednesday, while the journey of the truck that transported it is believed to have originated in Northern Ireland.

Belgium's federal prosecutor, Eric van der Sypt, told CNN that a people-trafficking investigation has been opened.

"At this point we don't know whether these people entered the container at (the Belgian port of) Zeebrugge or whether it arrived with them already onboard," van der Sypt told CNN, adding that the container arrived at 2:49 p.m. Tuesday and was shipped out of the port the same day.

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