GRAYS, England — Who were they, where were they going, and where were they from. It was all a mystery.

But one thing was heartbreakingly clear when the authorities peered into a container truck in a British industrial park early Wednesday morning: None of the 39 people inside were alive.

Their bodies were found after someone — officials have not said who — called an ambulance and the police came to investigate.

The passengers, all of whom save one teenager appeared to be adults, were declared dead at the scene. The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.