The bodies of 39 people have been found in a lorry container in Essex. The vehicle, found shortly before 01:40 BST at Waterglade Industrial Park in Eastern Avenue, Grays, came to the UK from Bulgaria, police said. Everyone inside the container, 38 adults and one teenager, died, Essex Police said. The lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder. Police said the vehicle entered the UK via Holyhead, Anglesey, on Saturday. Deputy Chief Constable Pippa Mills said identifying the victims remained a "number one priority", but was expected to be a "lengthy process". The National Crime Agency said it had sent officers to assist and identify any "organised crime groups who may have played a part". Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said if the lorry had come from Bulgaria, getting into Britain via Holyhead was an "unorthodox route". He added with reports of increased security checks at Dover and Calais "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". A spokesman for the Bulgarian foreign affairs ministry said it had not been confirmed whether the truck was registered in the country. "There is also no indication of the nationality of the human bodies found in the truck," he said. Police have appealed for witnesses and anyone with information about the lorry's route to contact them. Prime Minister Boris Johnson said it was an "unimaginable tragedy and truly heartbreaking". Speaking at Prime Minister's Questions, he said: "I know that the thoughts and prayers of all members will be with those who lost their lives and their loved ones. "I'm receiving regular updates. The Home Office will work closely with Essex Police as we establish exactly what has happened."