The bodies of 39 people have been found in a lorry container in Essex.
Police were called by the ambulance service shortly before 01:40 BST after the discovery at Waterglade Industrial Park in Eastern Avenue, Grays.
The lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.
Essex Police said the vehicle traveled from Bulgaria and entered the country via Holyhead, Anglesey, on Saturday.
Early indications are that 38 adults and one teenager have died, the force said. Police would not say whether any survivors were found in the lorry.
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.
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.”
“People trafficking is a vile and dangerous business,” Ms Doyle-Price tweeted, adding that she hoped Essex Police could “bring these murderers to justice”.