Police were called to Waterglade Industrial Park in Eastern Avenue, Grays shortly before 01:40 BST by the ambulance service after the discovery of a reported 39 dead bodies in a lorry container.
Per the BBC, the driver of the lorry, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.
The police force have advised that early indications appear to show that 38 adults and one teenager have died, but they did not say if any survivors had been found in the lorry.
Essex police stated that the vehicle in question travelled to England from Bulgaria, while Ch Supt Andrew Mariner said officers anticipated a "lengthy process" to identify the victims of the discovery;
"We have arrested the lorry driver in connection with the incident, who remains in police custody as our inquiries continue," he added, the BBC reports.
Meanwhile, Prime Minister Boris Johnson has said he is "appalled" by the "tragic incident".
"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 and their loved ones," he said.
Northern Ireland policy manager for the Freight Transport Association Seamus Leheny spoke about the "unorthodox" route the lorry appeared to have taken to enter Britain;
"People have been saying that security and checks have been increased at 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."
Home Secretary Priti Patel said in a statement that she's "shocked and saddened" by the incident;
“Essex Police has arrested an individual and we must give them the space to conduct their investigations. Home Office immigration officials will be working closely alongside them to establish how this horrific event came to happen." she continued.
