Police investigating the suspected mass murder of 39 people found in a lorry container on an industrial estate say the route a lorry has taken from Bulgaria will be a "key line of inquiry". 

The unidentified victims, including a teenager, were found dead inside the container - thought by experts to be a refrigerated unit - at the Waterglade Industrial Park in Grays early this morning. 

Police said the lorry driver, a 25-year-old man from Northern Ireland, has been arrested on suspicion of murder.

The lorry is from Bulgaria and entered the UK at Holyhead in north Wales, one of the main ports for ferries from Ireland.

The cargo was discovered more than 300 miles away shortly before 1.40am by ambulance staff. No further details have been provided.

Police have said tracking the route of the lorry "will be a key line of inquiry" amid concerns it may have made its way to the British mainland virtually unchecked by avoiding strict controls at Calais, in France, and on reaching England at Dover.

But haulage industry experts suggested the lorry was likely to have arrived in Ireland from Cherbourg or Roscoff, avoiding the tighter checks for people-smuggling at Calais and Dover.

Richard Burnett, chief executive of the Road Haulage Association, said: "This tragedy highlights the danger of migrant gangs people-smuggling on lorries."

He added: "It's highly unlikely that if this vehicle has come from Europe that it's been physically checked.

"Because of the migrant issue at Dover and Calais, you've got far more checks."

It is thought the lorry entered Britain via Dublin, which has the only direct ferry route to Holyhead.

Mr Burnett said the container appeared to be a refrigerated unit, where temperatures can be as low as -25C, and described conditions for anyone inside as "absolutely horrendous".

Seamus Leheny, Northern Ireland policy manager for the Freight Transport Association, said: "If the lorry came from Bulgaria, getting into Britain via Holyhead is an unorthodox route.

"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."