(CNN) -- UK police have launched a murder investigation after 39 people were found dead in a truck container at an industrial park in Essex, 30 kilometers east of London. One of the victims was in their teens.
Authorities believe the truck, which originated in Bulgaria, entered the UK through the Welsh port of Holyhead over the weekend. A regular ferry service connects Holyhead with the Irish capital, Dublin.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," Chief Superintendent Andrew Mariner said in a statement.
A 25-year-old Northern Irish truck driver was arrested "on suspicion of murder" after the bodies were discovered at the Waterglade Industrial Park in Grays early on Wednesday morning, Essex Police said.
"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue," Mariner added.
Deputy Chief Constable Pippa Mills told reporters that it was the police's "number one priority" to find out who the people who died were, and where they were from.
The force has set up a helpline for people to call if they are concerned their relatives might have been among those inside the truck.
Aerial footage of the scene, filmed by Reuters, showed a police cordon and black tent set up at the back of the white container, which was attached to a red Scania truck.
"This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened," Mariner said.
Police have placed a cordon around the area and say they are working with the local council "to mitigate against any impact our investigation scene will have locally."
PM 'appalled' by tragedy
News of the grisly discovery shocked British lawmakers; the local Member of Parliament speculated that the incident was linked to human trafficking. Police have not confirmed that claim.
Jackie Doyle-Price, who is the Conservative MP for Thurrock, where the bodies were found, wrote on Twitter: "Sickening news of 39 people found dead in a container in Grays.
"People trafficking is a vile and dangerous business. This is a big investigation for Essex Police. Let's hope they bring these murderers to justice."
Prime Minister Boris Johnson tweeted that he was "appalled by this tragic incident."
Speaking during the weekly Prime Minister's Questions sessions in the House of Commons, Johnson later told MPs that "the perpetrators of that crime" and "all such traders in human beings should be hunted down and brought to justice."
The UK Home Secretary, Priti Patel, said she was "shocked and saddened" by what she called the "utterly tragic incident," according to her Twitter account.
Interior Minister Brandon Lewis also called the news "tragic" in a tweet. "Thoughts are with victims, their families & friends. Police are working to ensure those responsible are brought to justice," he wrote.
The lengthy route the truck is believed to have taken to get into the UK has also raised eyebrows among haulage industry experts, who suggest it may have been attempting to circumvent security checks in the French port of Calais and the British port of Dover.
"If the lorry came from Bulgaria, getting into Britain via Holyhead is an unorthodox route," Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association (FTA), told PA news agency.
"People have been saying that security and checks have been increased in places like Dover and Calais, so it might be seen as an easier way to get in by going from Cherbourg or Roscoff, over to Rosslare, then up the road to Dublin.
"It's a long way around and it'll add an extra day to the journey," Leheny added.
Human trafficking incidents
Irish and Bulgarian authorities have been alerted to the discovery.
Irish Prime Minister Leo Varadkar told the country's Parliament on Wednesday that investigations would be carried out "if it's established the truck passed through Ireland."
Ireland's police force, An Garda Síochána, said it was monitoring the UK investigation, "and will provide every assistance possible."
The Bulgarian embassy in London has been in contact with British authorities, its Ministry of Foreign Affairs said in a statement.
"At present, it has not yet been confirmed whether the truck has a Bulgarian registration," the statement added.
In recent years there have been a number of fatal incidents in the UK and Europe involving large numbers of migrants who were found suffocated to death in trucks or containers.
Four human traffickers were given life sentences this summer over the deaths of 71 migrants whose bodies were found in an abandoned truck in Austria in 2015, Reuters reported.
In 2000, 58 Chinese migrants were found dead in a lorry in the southern British port of Dover.
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