The bodies of 39 people have been found in a lorry container in Essex, police have said.
One teenager was found dead alongside 38 adults at an industrial estate in the Thurrock area.
Police said they believed the lorry had come from Bulgaria and entered the UK at Holyhead, in north Wales, on Saturday. The port is one of the main entry points for ferries from Ireland.
Essex Police said it had launched a murder investigation after its officers were called to Waterglade Industrial Park, in Grays, in the early hours of Wednesday morning.
The 25-year-old lorry driver, who has been identified in media reports as Mo Robinson, from Northern Ireland, has been arrested on suspicion of murder.
There has been speculation that the suspect may have alerted the authorities himself.
The articulated vehicle, a red Scania model with a white refrigerated trailer and numerous floodlights attached to the cab, could be seen surrounded by forensic tents and temporary fencing on Wednesday.
Deputy Chief Constable Pippa Mills, of Essex Police, told a press conference that officers had not yet confirmed where the victims were from or their identities.
She said: “We are in the early stages of what is likely to be a lengthy investigation. It’s a complex scene.
“I’m unable to say at this stage how long the cordon will be in place. This is an absolute tragedy and a very sad day for Essex Police, and the local community.”
Ms Mills also appealed for anyone with information to call police on 101.
Officers were called to the industrial park at 1.40am on Wednesday by paramedics. Ms Mills would or could not answer about when and why paramedics were called to the industrial park, or by whom.
The industrial estate is currently closed and a witness who drove past just after 4am said that “all you could see was blue flashing lights”.
Essex Police is investigating alongside the Home Office’s Border Force and the National Crime Agency, home secretary Priti Patel told the Commons.
The Independent has contacted the Bulgarian embassy in London for comment.
A witness said the lorry had been in the industrial estate for at least about 12 hours before police were called.
Andy Larkin, manager at Bronze Mechanical Handling, a nearby mechanics company, said he saw the lorry on Tuesday at about 2pm when he walked to meet a customer.
He told The Independent: “What made me notice it is it had those big chrome American exhaust pipes. The curtains in the cab were closed, as though the driver was asleep in there.
“Until about a month ago, there was a refrigeration company there and you used to get loads of lorries from all over Europe parked around there waiting to get in.
“It wouldn’t have looked out of place when that was open. But it’s unusual for them to be there now. That’s road is usually empty, apart from cars.”
Mr Larkin said he was reviewing footage from his CCTV cameras.
The 39 deaths sparked condemnation of the trade in trafficked people by MPs.
During prime minister’s questions, Thurrock MP Jackie Doyle-Price said: “To put 39 people into a locked metal container shows a contempt for human life that is evil. The best thing we can do in memory of those victims is to find the perpetrators and bring them to justice.”
Responding, Boris Johnson said: “I must say I do share her strong desire now for the perpetrators of that crime, and indeed all those who engage in similar activity – because we know that this trade is going on – all such traders in human beings should be hunted down and brought to justice.”
Andrew Mitchell called on Ms Patel to push for a new United Nations convention to tackle the “modern equivalent of the slave trade”, citing smugglers preying on migrants and refugees in the Mediterranean and Libya, as well as Wednesday’s events.
“He is right that as the world has changed, conflict has changed,” Ms Patel said. “We’re seeing all sorts of desperate situations around the world and there is much more that we can do in terms of leveraging our own voice and our own influence with the big organisations.”
Pressure groups laid the “ultimate responsibility” at the feet of government.
Stephen Hale, chief executive of Refugee Action, said in a statement: “This appalling tragedy shows the urgent need for the government to create safe and legal routes to the UK for people fleeing war and persecution. The lack of these routes is forcing desperate people to put their lives into the hands of smugglers.”
And Satbir Singh, of the Joint Council for the Welfare of Immigrants, said: “Nobody should be in any doubt that the ultimate responsibility for these deaths lies with government policy which has deliberately closed down safe and legal routes into Britain.
“We need a commitment to opening safe and legal routes to the UK, and quick decisions on applications from people seeking to make a better life here.”
A spokesman for the Road Haulage Association said: “This tragedy highlights the danger of migrant gangs people-smuggling on lorries.”