Police in southeastern England said 39 people were found dead Wednesday inside a truck container believed to have come from Bulgaria.
The truck, which is said to have entered Britain via the Welsh port of Holyhead on Saturday, was found by ambulance workers at Waterglade Industrial Park, Grays.
"This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened," Essex Police Chief Superintendent Andrew Mariner said.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process."
A 25-year-old-man from Northern Ireland has been arrested on suspicion of murder. He remains in custody.
A cordon has been put in place and access to and from the industrial park remains closed.
"We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally," Mariner said.
Prime Minister Boris Johnson said in a tweet he was appalled by what had happened and gave assurances that work was underway to find the cause of the incident.
I’m appalled by this tragic incident in Essex. 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 & their loved ones.— Boris Johnson (@BorisJohnson) October 23, 2019
Home Secretary Priti Patel also said she was “shocked and saddened” by the news.
Shocked & saddened by this utterly tragic incident in Grays. Essex Police has arrested an individual and we must give them the space to conduct their investigations.— Priti Patel MP (@patel4witham) October 23, 2019