Thirty-nine people, including a teenager, have been found dead inside a lorry container at an industrial park in Essex.
Police were called by the ambulance service after the discovery at Waterglade Industrial Park in Grays at 1.40am.
All 39 people were pronounced dead at the scene and the lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.
The vehicle is believed to have come from Bulgaria and entered the UK at Holyhead in north Wales on Saturday.
Essex Police Chief Superintendent Andrew Mariner said: “This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened.
“We are in the process of identifying the victims, however I anticipate that this could be a lengthy process.
“We believe the lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate. We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue.”
Police are appealing for anyone with information to get in touch with them, and have put a cordon in place and access to and from the industrial park is closed.
Mr Mariner added: “I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible. We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally.”
Deputy Chief Constable Pippa Mills told reporters outside Grays police station that the incident was an “absolute tragedy and a very sad day for Essex Police and the community”.
National Crime Agency officers have been deployed to assist Essex Police with the investigation, and three Red Cross emergency support vehicles were also seen heading into the site.
The Bulgarian foreign affairs ministry has said it had “no information” about the incident prior to UK media reports.
It told Bulgarian newspaper Dnevnik: “Our embassy is in contact with local authorities and is reviewing the case.”
Prime Minister Boris Johnson tweeted: “I’m appalled by this tragic incident in Essex. I am receiving regular updates from the Home Office & will work closely with Essex Police as we establish exactly what has happened.