The 39 bodies were found in a lorry container in eastern England on 23 October.
The bodies of 39 people, believed to be Chinese nationals, have been found in a lorry container in Essex in eastern England.
The driver has been arrested and a murder investigation has been launched.
Essex Police said they were working "to piece together the circumstances of this horrific event", which they say is the largest murder investigation in the force's history.
Here's what we know so far about the discovery.
Who were the victims?
Essex police said they believed all 39 victims to be Chinese nationals.
All were thought to be adults except one young adult woman, who police previously said may have been a teenager.
Eight of the deceased were women and 31 were men, police confirmed.
BBC Monitoring says the overriding reaction in China is one of shock.
Officials from the Chinese embassy in London have said they are on their way to the scene to verify the situation, Monitoring adds.
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".
All of the dead will undergo a coroner's examination to establish the cause of death.
Police will then attempt to identify each individual, but have warned this will be a "substantial operation" and they cannot estimate how long it will take.
Prime Minister Boris Johnson said the Home Office would work closely with Essex Police "as we establish exactly what has happened".
"My thoughts are with all those who lost their lives and their loved ones," he posted on Twitter.
What about the lorry?
Essex Police said the tractor unit (the front part of the lorry) had entered the country via Holyhead - an Irish Sea port in Wales - on Sunday 20 October, having travelled over from Dublin.
They believe the lorry then collected the trailer, which arrived in Purfleet on the River Thames from Zeebrugge, Belgium, at about 00:30 BST on Wednesday (23:30 GMT Tuesday).
The cab and trailer then left the port shortly after 01:05.
Police were called to the industrial park where the bodies were discovered shortly before 01:40.
Officials in Belgium are investigating how long the container spent there before travelling to the UK.
It had stickers on the windscreen saying "Ireland" and "The Ultimate Dream".
Media caption"An absolute tragedy and a very sad day for Essex" – Pippa Mills from Essex Police
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.
Police in Northern Ireland have carried out raids on two houses associated with Mr Robinson - one in Markethill, County Armagh, and another in nearby Laurelvale.
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 National Crime Agency said it is trying to identify any "organised crime groups who may have played a part".
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.
Mr Burnett told the BBC that ports at Calais and Coquelles use CO2 monitors, sniffer dogs and scanners to check for people smuggling.
"That kind of pushes the problem further out to more remote ports," he said. "If we haven't got the infrastructure there from a security perspective to check those vehicles, then traffickers will definitely use those routes to get migrants into the UK."
Britain's National Crime Agency told the BBC that all UK ports were being used for people smuggling.
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.
Why could Bulgaria be significant?
Since the completion of a 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.
Media captionHas Bulgaria become the new route of choice for migrants trying to enter Europe?
One part of the smugglers' network hands them on to others.
In 2015, the bodies of 71 people were found in an abandoned lorry on an Austrian motorway. Police suspected the vehicle was part of a human trafficking operation. An Afghan and three Bulgarians received long prison sentences for people smuggling, with 10 other accomplices, almost all of them Bulgarians, being jailed for shorter terms.
How many migrants have died in transit in the UK?
The number of migrants who die in transit has been recorded by the UN since 2014.
Since then, five bodies of suspected migrants have been found in lorries or containers in the UK:
- 2014: An Afghan migrant was found dead at Tilbury Docks, Essex. He was in a shipping container along with 34 other Afghans who survived
- 2015: Two migrants were found dead in a wooden crate in a warehouse in Branston, Staffordshire. The crate had been sent from Italy
- 2016: An 18-year-old migrant was crushed while clinging to the underside of a lorry in Banbury, Oxfordshire
- 2016: A body was found in Kent in the back of a lorry which had travelled from France
In 2000, 58 Chinese migrants were found suffocated in a lorry at the southern English port of Dover.