The immigrants rescued from a shipping container were probably already inside when it was dropped at a European port before setting sail for Britain, Belgian police believe.
International agencies are working alongside British Police as they continue their investigation into how the group - including one man who later died - came to arrive at the Port of Tilbury early yesterday morning.
Today Peter De Waele, spokesman for the Federal Police in Belgium, said it appeared to be "impossible" for the 35 men, women and children to be loaded into the unit in the time it was at Zeebrugge, a port in the north of the country. He said investigators have been combing CCTV and are "very hopeful" that they can track down the driver and company of the vehicle that deposited the container.
De Waele told the Press Association each container is captured on camera and their seals are subject to "very strict" controls. He said: "We saw that this container was there for one hour in all in an area where there is a lot of cameras, a lot of people and a lot of cars driving. It was around 6pm so it wasn't dark.
"We think that the possibility [is that] the victims were already on the container before it was put down in Zeebrugge because in that area it is rather impossible to put 35 people in that container. That is not exact information. I think the investigation has to give answers to this question."
He declined to confirm whether detectives have already identified the vehicle which delivered the container to Zeebrugge.
But he added: "My colleagues told me that they were very hopeful looking at the pictures that they [could] find the company and also the driver who put the container in Zeebrugge. It is too early saying the driver is involved but when we find the driver we can work backwards."
De Waele added that they are working closely with Essex police and described tackling human trafficking as a "priority" in Belgium. The container was one of around 50 on board the P&O Ferries commercial vessel the Norstream when it arrived in Essex.
The immigrants, all thought to be from the Indian subcontinent, were rescued after port authorities heard banging and screaming from inside the container at around 6.30am yesterday. One man died and the others were taken to hospital suffering from hypothermia and dehydration.
Police in Essex, who are treating the incident as a "homicide" investigation, are expected to begin questioning members of the group today.