Police were attempting on Wednesday to identify the bodies of 39 people found dead in a truck trailer in Essex, in one of the worst losses of life involving an apparent people-smuggling operation in the UK.
The latest incident is likely to evoke comparisons with others around Europe in which migrants have suffocated after hiding in trucks. The loss of life is the highest in the UK since 2000, when 58 people were found dead near Dover in the back of a truck that had arrived from the Netherlands.
British authorities were trying to piece together the movements of the trailer, which arrived at the port of Purfleet in Essex from Zeebrugge in Belgium in the early hours of Wednesday, shortly before being picked up by a Bulgarian-registered tractor unit that had come from Northern Ireland.
The bodies were discovered just after 1am when an ambulance crew called in the police to investigate the trailer after the lorry had stopped about a mile from the port on an industrial estate near Grays in Essex, 23 miles east of London. The truck driver, a 25-year-old man from Northern Ireland, was arrested on suspicion of murder.
Pippa Mills, deputy chief constable of Essex Police, said the identification of the victims was the "number one priority". She said the nationality of the victims, thought to be 38 adults and a teenager, was also unknown.
There was some initial confusion about how the trailer had entered the UK after the police initially said the truck had entered the UK on Saturday via Holyhead, in Wales. Subsequently, the force said the trailer unit had arrived in Essex separately from Belgium, while the tractor unit had "originated" from Northern Ireland.
Bulgarian authorities said the tractor unit was registered in the city of Varna under a company owned by an Irish citizen, according to Reuters. Bulgarian broadcaster BNR reported that unnamed sources said the truck had left Bulgaria in June 2017 and had not returned.
"This will be a lengthy and complex investigation and we continue to work with local partners and international authorities to gather vital intelligence and identify those who have sadly died," Essex Police said in a statement.
The deaths raise fresh questions about scrutiny of cross-Channel freight movements. Border Force officers at large ports such as Dover now routinely deploy sniffer dogs and carbon dioxide detecting equipment to discover migrants hidden inside trailers entering the UK from continental Europe. It was unclear on Wednesday whether similar equipment was deployed at the far less busy, freight-only ferry terminal at Purfleet.
The National Crime Agency, which investigates organised crime, said it was working the police and immigration officials "to urgently identify and take action against any organised crime groups who have played a role in causing these deaths."
Priti Patel, the home secretary, tweeted that she was "shocked and saddened" by the "utterly tragic" incident, while Boris Johnson, prime minister, said he was "appalled" and was receiving regular updates.
"The Home Office will work closely with Essex police as we establish exactly what has happened," Mr Johnson said. "My thoughts are with all those who lost their lives and their loved ones."
However, the Joint Council for the Welfare of Immigrants, a charity, called the statements from Ms Patel and Mr Johnson "empty expressions of sadness and shock" and said the government should commit to opening "safe and legal routes" for migration.
"Nobody should be in any doubt that the ultimate responsibility for these deaths lies with government policy, which has shut down safe and legal routes to the UK," the group said.