British police found the bodies of 39 people in a truck container at an industrial estate to the east of London on Wednesday.
Police were called by the ambulance service shortly after the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.
Police said the truck was believed to have come from Bulgaria and to have entered Britain at Holyhead in Wales on Saturday. The driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder, police said.
The 39 victims, 38 adults and one teenager, were all pronounced dead at the scene.
Ch Supt Andrew Mariner said officers were trying to identify victims but anticipated a “lengthy process”.
“We have arrested the lorry driver in connection with the incident, who remains in police custody as our inquiries continue,” he said.
Prime Minister Boris Johnson said he was “appalled by this tragic incident”.
He said: “I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened. My thoughts are with all those who lost their lives and their loved ones.”
