“We would have never expected such tragedy to happen here, in this zone,” said Ryan Sharpe, a 24-year-old driver at Transmec Group, a nearby freight company.

“Take any industrial estate you’d find in the U.K. and in Europe,” said Catalin Bortoc, a 28-year-old employee at an auto parts firm just across from where the truck was being examined. “Except you add a lorry with 39 bodies inside. It’s just so awful.”

Britain has long been a destination for migrants, and smugglers have often used the English Channel crossings to traffic people into the country.

Most covert transport methods pose significant safety risks, according to Britain’s National Crime Agency. Migrants often travel in shipping containers or commercial vehicles that are transported by truck, rail or ferry, or they cross on small boats.

Organized gangs frequently smuggle people in hard-sided trucks like the one in Essex, while small-time traffickers tend to use soft-sided trucks.

The crime agency has acknowledged that the length of the Britain’s coastline and the sheer volume of passenger and freight traffic into the country make identifying illegal migration “a significant challenge.”

Elian Peltier reported from Grays, England, and Megan Specia from London. Benjamin Mueller contributed reporting from London, Boryana Dzhambazova from Sofia, Bulgaria, and Ed O’Loughlin from Dublin.