Driver arrested after 39 bodies found in tractor-trailer - Essex police.

The driver of the Bulgarian-registered vehicle, which appears to have traveled via Dublin, is being held on suspicion of murder.

The vehicle is believed to have originated from Bulgaria and entered the United Kingdom through Holyhead port in Wales on Oct. 19, police said.  The U.K. Home Office, the government department in charge of domestic security and immigration, will be working with Essex police authority on the investigation.

"This is a tragic incident where a large number of people have lost their lives," Essex Police Chief Superintendent Andrew Mariner said in a statement. "Our inquiries are ongoing to establish what has happened. We are in the process of identifying the victims, however I anticipate that this could be a lengthy process."

"We have arrested the lorry driver in connection with the incident who remains in police custody as our inquiries continue," he added.

The Bulgarian Ministry of Foreign Affairs confirmed that the truck was registered in Bulgaria on behalf of a company owned by an Irish citizen.

At a press conference Wednesday morning, Constable Pippa Mills of the Essex police force said the “identification of the victims remains our number one priority” and called for anyone with information about the vehicle to come forward.

British Prime Minister Boris Johnson described the news as "tragic" and said he was receiving regular updates from the authorities as they piece together what happened.

"I’m appalled by this tragic incident in Essex," he wrote on Twitter. "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."