Police are urgently trying to track down the driver who delivered the trailer, later discovered to contain 39 people, to a port in Belgian before being shipped to the UK.
The Belgian prosecutor’s office said officers were attempting to find the driver who delivered the refrigerated container to Zeebrugge on Tuesday.
Essex Police were called to the Grays Waterglade Industrial Park in Grays just before 1.40am on Wednesday.
Thirty-nine people, initially believed to be Chinese nationals, were found dead inside the container. Missing person reports now suggest that a number of the victims may have been from Vietnam, however formal identification has not yet taken place.
A police spokesperson told PA news agency: “We’re trying to identify the driver.”
He said Belgian authorities were also working to “track the route of the container” and find anyone responsible for “collaborating with the transport”.
“We would like people to be arrested as soon as possible,” he added.
Four people have been arrested in the UK in connection with the incident, including the driver found with the truck in Essex, 25-year-old Mo Robinson from Northern Ireland who was held on on suspicion of murder on Wednesday and has remained in custody since.
A 48-year-old man from Northern Ireland was taken into custody on Friday evening on suspicion of conspiracy to traffic people and on suspicion of manslaughter, hours after a couple named locally as haulage boss Thomas Maher and his wife Joanna, both 38, from Warrington were arrested on suspicion of 39 counts of manslaughter and people trafficking.