Video report by ITV News Correspondent Dan Rivers
A murder investigation is under way following the discovery of 39 bodies in a lorry trailer parked on an industrial estate in Essex.
Early indications suggested that all the victims bar one was an adult, with one thought to be a teenager.
Essex Police said it was called by the ambulance service shortly before 1.40am on Wednesday following the discovery of a lorry trailer with people inside at Waterglade Industrial Park, Eastern Avenue, Grays.
All of the 39 victims were found dead at the scene. Police have not said how many were men or women.
A 25-year-old man from Northern Ireland has been arrested and is understood to be Mo Robinson from County Armagh.
Authorities believe the lorry arrived in the UK from Zeebrugge in Belgium to Purfleet, Essex, and docked in the Thurrock area shortly after 12:30am on Wednesday.
The tractor unit is believed to have originated in Northern Ireland. The lorry has been transported by forensic police to a secure location where the crime scene can be examined.
Exclusive CCTV footage obtained by ITV News shows the lorry in which the 39 bodies were later found.
The video shows the moments before the vehicle turned on to the industrial estate where the discovery was made.
The lorry was captured at around 1.24am driving down Motherwell Way.
A second camera captured the lorry from a different angle on the same road before it turned on to the industrial park.
An Essex Police statement said: "Originally, we reported that the lorry had travelled into the country through Holyhead on Saturday 19 October.
"After further enquiries, we now believe that the trailer travelled from Zeebrugge into Purfleet, and docked in the Thurrock area shortly after 12:30am.
"The tractor unit of the lorry is believed to have originated in Northern Ireland.
"We believe the lorry and trailer left the port shortly after 1.05am."
The driver of the lorry remains in custody having been arrested on suspicion of murder.
The police statement added: "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."
At a press conference, Pippa Mills, deputy chief constable of Essex Police, said the dignity of those who had died was imperative.
She said the incident had attracted both national and international attention but that the focus of the police was conducting the operation with the "utmost respect" for the victims.
Ms Mills added that the lorry and trailer would be moved to a secure location at Tilbury Docks to allow the bodies to be removed.
She said they had yet to identify the victims or their nationalities.
"We must manage this sensitively with their families," she said.
In a statement, the East of England Ambulance Service NHS Trust (EEAST) said it had responded to a call to scene - though it did not say who called it in.
The service sent five ambulances, hazardous area response teams and a car from the Essex and Herts Air Ambulance to the scene.
Its statement added: "Unfortunately, 39 people had died prior to our arrival."
Aerial footage taken earlier on Wednesday showed the red truck still coupled to its white trailer, parked on a side street on the industrial estate.
A number of emergency vehicles were at the scene. There are tented areas and green screens have been erected at the back of the trailer to shield the view at street level.
Bulgarian authorities said they could not yet confirm that the truck had started its journey from Bulgaria.
"We are in contact with our embassy in London and with British authorities," foreign ministry spokeswoman Tsvetana Krasteva said.
The Bulgarian foreign ministry later confirmed the Scania truck was registered in Varna (on the east coast of Bulgaria) under the name of a company owned by a female Irish citizen.
Richard Burnett, chief executive of the Road Haulage Society, said "this tragedy it highlights the danger of migrant gangs people-smuggling on lorries".
He said: "It's highly unlikely that if this vehicle has come from Europe that it's been physically checked.
"Because of the migrant issue at Dover and Calais, you've got far more checks."
Mr Burnett said the container appeared to be a refrigerated unit, where temperatures can be as low as -25C – and described conditions for anyone inside as “absolutely horrendous”.
Prime Minister Boris Johnson issued a brief statement before the start of PMQs describing the incident as an "unimaginable tragedy and truly heartbreaking".
During PMQs, 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, Mr Johnson said it was difficult to imagine what the emergency services witnessed when they opened the doors to the trailer.
He 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."
Irish premier Leo Varadkar said any necessary investigations would be undertaken if it was established the lorry had passed through Ireland.
"The information that we have so far this morning is very sketchy but there are some reports that the truck may have passed through Ireland at some point," he told the Dail parliament in Dublin.
"So, obviously we are going to need to get more information and carry out any necessary investigations that may be required.
"It is a real terrible and human tragedy, and we will carry out any investigations that are necessary if it's established that the truck did pass through Ireland."
Paige Wade was driving past the industrial park at 4.15am on the way home from work.
She told the PA news agency: “I had just finished work while driving up Motherwell Way and all you could see was blue flashing lights, as I was driving past there was police tape to cordon off the start of Eastern Avenue.
“I knew it was serious because of how many police cars and ambulances were there, but the police had parked their cars across the whole access of the road so you couldn’t see anything.
“There’s always lorries around there as they park up there for the night.”
The exact scale of the people-smuggling problem is unknown and the National Crime Agency is yet to publish statistics on most of its work in this area.
But it does say the number of potential victims of human trafficking and modern slavery reported to the National Referral Mechanism (NRM) - has continued to rise.
Last year, there were 6,993 cases - a 36% increase on 2017.
The discovery in Essex is not the worst of its kind in the UK.
Seven men were jailed by a Dutch court for their role in the human smuggling operation that led to the young people suffocating and the Dutch lorry driver was jailed for 14 years.