The driver of the truck, identified as Morris Robinson, 25, from Northern Ireland, is still being held for questioning by the police on suspicion of murder. He has yet to be charged.

The case bears all the marks of a smuggling operation, officials and experts have said, and has links to Britain, China, Belgium, Ireland, Bulgaria and possibly Vietnam. The British authorities were scrambling to investigate who the victims were, who had facilitated their journey and what exactly their movements were.

On Thursday, the British authorities began removing bodies from the truck, which had been driven to a secure site so that the victims could be removed in a manner that preserved their dignity, officials said.

The bodies of 11 victims were taken to a hospital to undergo post-mortem examinations, the Essex Police said in a statement, adding that the process was likely to take some time.

A Twitter post on Friday by the rights activist Hoa Nghiem revealed that the family of a Vietnamese woman, Pham Thi Tra My, 26, feared she might have been in the trailer. Her younger brother, Pham Manh Cuong, reached by phone on Friday, said his sister had traveled from Vietnam to China in early October, before flying to France.

From there, Mr. Pham said, she attempted to travel to Britain but had been stopped by the police and returned to France. Her relatives said they believed she had then made a second attempt to travel to Britain: They received a frantic message from her around 10:30 p.m. British time on Oct. 22.