LONDON — British police launched one of the country’s biggest ever murder investigations on Wednesday after 39 bodies were found inside a tractor-trailer at an industrial park in southeast England.

Essex police said the driver, a 25-year-old man from Northern Ireland, had been arrested on suspicion of murder.

The bodies — 38 adults and one teenager — were found at Waterglade Industrial Park in Grays, about 25 miles east of central London. Police have not yet offered an account of what might have happened, but the scene bore the markings of human trafficking.

Essex Deputy Chief Constable Pippa Mills said police had not yet identified the dead or where they are from, adding that it could be a “lengthy process.” 

Essex police said they thought the truck traveled from Zeebrugge, a Belgian port, to Purfleet, a small town in Essex on the River Thames, docking shortly after 12:30 a.m. That account is an update from an earlier police report that the truck entered the United Kingdom on Saturday via Holyhead in North Wales.

At about 1:05 a.m. Wednesday, the truck left Purfleet, police said. Thirty-five minutes later, police received a call from local ambulance services saying they discovered the container. It was unclear how the ambulance services had been alerted.

“We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue,” Essex Chief Superintendent Andrew Mariner said in the statement. “This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened.”

Police on Wednesday did not name the driver, though several British media outlets identified him, citing sources in Northern Ireland, and posted photos from what were said to be his social media accounts.

The truck was registered in Varna, Bulgaria — a port city on the Black Sea — to a company owned by an Irish citizen, according to a statement by the Bulgarian foreign ministry. Bulgarian Prime Minister Boyko Borissov told a local television broadcaster that the truck left immediately after it was registered in 2017 and hadn’t returned.

British Prime Minister Boris Johnson said he was “appalled by this tragic incident in Essex.”

“I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened. My thoughts are with all those who lost their lives & their loved ones,” he tweeted.

Jackie Doyle-Price, a Conservative lawmaker, told Parliament on Wednesday: “Sadly, this is not the first time that we have found people in metal containers in my constituency. We’re really sorry to say it’s all too regular an occurrence and it was only a matter of time before that would end in tragedy.”

“This is now a multinational problem that we need to fix,” she added.

British authorities say human trafficking and modern-day enslavement are on the rise. National Crime Agency figures show that nearly 7,000 possible victims were reported last year — a 36 percent increase from 2017. Those people came from 130 countries, with Albanians and Vietnamese the most common foreign nationalities.

Police didn’t identify the people found Wednesday as migrants, but the case echoed fatal incidents involving migrants smuggled in containers.

In June 2000, the bodies of 58 Chinese immigrants were found in the back of a truck container in the English port city of Dover. The following year, a Dutch driver was sentenced to 14 years in jail for manslaughter. The immigrants, who paid a smuggling gang $26,000, suffocated after the driver closed a vent on the truck during a five-hour ferry ride across the English Channel.

In August 2015, 71 bodies were found on a highway in Austria, inside a sealed and locked freezer truck. Most of the people were from Syria, Iran, Iraq and Afghanistan. The discovery came at the peak of Europe’s refugee influx and became one of its defining, tragic moments.

Research by German public television and the Sueddeutsche Zeitung newspaper later revealed that Hungarian officials had tapped the traffickers’ phones but failed to act in time. 

After the 2015 incident, the E.U. law enforcement agency Europol added a dedicated European Migrant Smuggling Center. In a report published this year, the center found that the most common method of smuggling involves hiding people inside vehicles.

Rod McKenzie, managing director of policy and public affairs at the Road Haulage Association, said the journey for the people in the truck found in Essex would have been “hellish.” He said it was clear from pictures that the truck had a refrigerated unit, where temperatures can go as low as minus-13 Fahrenheit.

“It would be completely dark, probably completely airless, no sanitary facilities, possibly freezing temperatures, with the likelihood of death from freezing or suffocation enormous,” McKenzie said.

He surmised that those who sent the trailer to Essex may have chosen the route in an effort to avoid the strict checks at the popular crossing between Calais, France, and Dover. He said authorities there use sniffer dogs and monitors that can detect heartbeats, heat and CO2 levels, among other things.

“Purfleet, however, doesn’t have that level of technology to screen lorries,” he said.

Noack reported from Berlin. William Booth in London contributed to this report.