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 in Grays, a town just by the River Thames around 25 miles (40 kms) east of central London.

“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. He has not been charged or identified.

A cordon has been put in place and access to and from the industrial park remains closed.

“We are working with Thurrock Council (the local authority) to mitigate against any impact our investigation scene will have locally.” Mariner said.

Prime Minister Boris Johnson pledged in a tweet to work closely with Essex Police to establish exactly what happened.

“My thoughts are with all those who lost their lives & their loved ones,” he said.

Police have not formally linked the deaths to people trafficking but a link is assumed because of the way the victims were crammed into the truck container.

Jackie Doyle-Price, who represents Thurrock in parliament, tweeted that “people trafficking is a vile and dangerous purpose … Let’s hope they bring these murderers to justice.”

Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said the route that had been used was “unorthodox” since it apparently involved travelling to the island of Ireland and then entering Britain via a ferry to a major passenger port in Wales.

He said that choice may have been influenced by increased security and checks in the major English port of Dover and the French port of Calais.

“People have been saying that security and checks have been increased at places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin,” he said.

“It’s a long way around and it’ll add an extra day to the journey.”