The bodies of 39 people have been found in a lorry container in Essex in eastern England.
The driver has been arrested and a murder investigation has been launched.
Here's what we know so far about the tragic discovery.
Who were the victims?
Essex police said in a statement that it had received first reports about bodies found in a lorry at the Waterglade Industrial Park in Grays just before 01:40 local time (00:40 GMT) on Wednesday. The town is about 15km (nine miles) east of London.
The bodies were discovered by ambulance staff, and the police later said that "sadly all 39 people inside the container had died".
"Early indications suggest that one of these people was a teenager, the rest are believed to be adults," the police said.
The nationalities of the victims were not immediately known, with the police warning that the formal identification "could be a lengthy process".
The police had earlier suggested that the lorry could be from Bulgaria, but later said they believed the vehicle entered the UK from Belgium.
The Bulgarian foreign ministry said in a statement: "The bodies of 38 adults and a teenager have not yet been identified, but police said that it is highly unlikely that they are Bulgarians."
What about the lorry?
Essex police believe the trailer arrived in Purfleet on the River Thames from Zeebrugge, Belgium, at about 00:30 BST on Wednesday (23:30 GMT Tuesday).
The tractor unit (the front part of the lorry) is thought to have come from Northern Ireland and picked up the trailer from Purfleet, Essex, shortly after 01:05 (00:05 GMT).
Belgian prosecutors have launched an investigation to establish whether the trailer travelled through Belgium.
Essex police corrected their earlier statement that said the lorry had entered the UK at Holyhead, a major Irish Sea port in Wales, on 19 October.
However, Irish Prime Minister Leo Varadkar said investigations would be undertaken to establish if the lorry had passed through Ireland.
In the statement, Essex police said "the tractor unit [front section] of the lorry is believed to have originated in Northern Ireland".
In the earlier statement, the police had said they believed the lorry was from Bulgaria.
The Bulgarian foreign ministry said: "The Scania truck was registered in Varna (on the east coast of Bulgaria) under the name of a company owned by an Irish citizen."
Bulgarian officials were also quoted as saying that the lorry was last in Bulgaria in 2017.
It was not immediately known where the container originated from.
Richard Burnett, chief executive of the Road Haulage Association, said the container appeared to be a refrigerated unit where temperatures could be as low as -25C (-13F).
Later on Wednesday, the lorry was moved to a secure site so the bodies could be "recovered while preserving the dignity of the victims", Deputy Chief Constable of Essex Police Pippa Mills said.
And the driver?
The driver was named locally as Mo Robinson, 25, from the Portadown area of County Armagh, Northern Ireland.
He has been arrested on suspicion of murder, and is being questioned by police.
Was it an attempt to smuggle people into the UK?
We do not know at this stage, and Essex police warn that the investigation will be "lengthy and complex".
The BBC's home editor Mark Easton reports that people smugglers have increasingly moved to other routes since the Calais migrant camps were shut three years ago in France and security measures were increased at Dover and the Channel Tunnel.
Britain's National Crime Agency told the BBC that all UK ports were being used.
More dangerous methods are being used to get human cargo through.
The most common one is still being hidden in the back of a lorry, but increasingly commercial shipping containers are being used, sometimes even refrigerated ones.
Risks are substantial for the migrants, who can pay £10,000 ($12,900) or more for a space on these vehicles.
A lorry is charged just over £400 for a ferry crossing from Zeebrugge to Purfleet.
Why could Bulgaria be significant?
Since the completion of the fence on the Bulgaria-Turkey border in 2016, most asylum seekers trying to reach Western Europe do so hidden in trucks, the BBC's Nick Thorpe reports.
One part of the smugglers' network hands them on to others.