London, Oct. 24 -- Police in southeastern England launched a murder investigation after 39 people were found dead early Wednesday inside a large cargo truck that authorities believe was registered in Bulgaria and came into the country via Ireland.

The truck, which British police said entered the U.K. on Saturday via the Welsh port of Holyhead, was found across the country at the Waterglade Industrial Park in Grays, a town 25 miles (40 kilometers) east of London on the River Thames.

Police did not formally tie the deaths to human trafficking but a link was assumed because of the way the victims were crammed into the truck's container. The truck's apparently circuitous route into and around Britain also raised suspicions among shipping ex...