A shipping container from Zeebrugge, Belgium, unloaded at a dock in eastern England Saturday contained 35 people, including children, police said.
One man has died while others are being treated for dehydration and hypothermia at local hospitals.
The survivors are believed to be from the Indian subcontinent, BBC News reported.
"As a result of the ferry docking, staff at the port became aware of screaming and banging coming from the container," Essex Police Superintendent Trevor Roe said.
It is unclear where the container originated.
Authorities launched a homicide investigation into the death. About 50 other containers are being searched.
"This is a humanitarian issue and the welfare of these patients is a priority," Roe said.
Investigators are working with their Belgian counterparts.
Authorities are working to identify "the gangs or whoever may be involved in this conspiracy to bring these people in this way over to this country,” Roe added.
Image credit: Wikimedia Commons / Captain Albert E. Theberge, NOAA Corps