The bodies of 39 people, believed to be Chinese nationals, have been found in a lorry trailer 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 in a statement they believed all 39 victims – eight women and 31 men – to be Chinese nationals.

All were thought to be adults except one “young adult” woman, who police previously said may have been a teenager.

The police said they 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”.

Police began the process of moving the bodies on Thursday. Eleven victims were taken by private ambulance from the Port of Tilbury to Broomfield Hospital in Chelmsford.

All of the dead will undergo a coroner’s examination to establish the cause of death.

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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.

What has Chinese reaction been so far?

BBC Monitoring says the overriding reaction in China is one of shock.

However, the only official comments have so far come from the Chinese embassy in London, which said on Wednesday its staff were “driving to the scene to verify the situation”.

Given China has only just become aware of the update that the bodies may be Chinese nationals, the scale of reaction has been huge, BBC Monitoring says.

Already, more than 100,000 users of the popular Sina Weibo microblog have used the hastag #39BodiesInBritishTruckWereChineseNationals.

The overriding sentiment from users is shock, given that no known similar incident has taken place in over a decade, and no-one is aware of a large group of Chinese nationals being unaccounted for.

The last known similar incident took place in June 2000, when the bodies of 58 Chinese people were found in a lorry in the English port of Dover. There were two survivors.

What about the lorry and the trailer?

Essex Police said the tractor unit (the front part of the lorry) had entered the UK via Holyhead – an Irish Sea port in Wales – on Sunday 20 October, having travelled over from Dublin, the Republic of Ireland.

They corrected a previous statement which stated that this had taken police on the 19 October.

The unit had stickers on the windscreen saying “Ireland” and “The Ultimate Dream”.

Police believe the tractor unit then collected the trailer, which arrived in Purfleet on the River Thames from Zeebrugge, Belgium, at about 00:30 local time on Wednesday.

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 trailer spent there before travelling to the UK.

On Thursday, Global Trailer Rentals, a Dublin-based company, confirmed it owned the refrigerated trailer in which the bodies were found, according to the Irish Broadcaster RTE.

Global Trailer Rentals said the trailer had been leased on the 15 October from its yard in County Monaghan.

In a statement, the company added that it was “entirely unaware that the trailer was to be used in the manner in which it appears to have been”.

Richard Burnett, chief executive of the Road Haulage Association, said the trailer appeared to be a refrigerated unit where temperatures could be as low as -25C (-13F).

The lorry now is at a secure site in Essex.

Essex police had previously 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.

And the driver?

The driver was named locally as Mo Robinson, 25, from the Portadown area of County Armagh, Northern Ireland.

He was arrested on suspicion of murder on Wednesday.

On Thursday, police were granted an extra 24 hours to question him.

Three properties in Northern Ireland have been raided and the National Crime Agency is working to establish if “organised crime groups” were involved.

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.

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A lorry is charged just over £400 for a ferry crossing from Zeebrugge to Purfleet.

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

Data was not collected in the same way before the migrant crisis began in 2014, but such deaths are not new.