Police are continuing to question a Northern Ireland man over 39 bodies which were found in a lorry container in Essex.
The 25-year-old was arrested on Wednesday on suspicion of murder.
The PSNI have carried out two house searches in Northern Ireland in relation to the arrest.
Early indications suggest 38 of the victims are adults and one is a teenager.
Forensic experts have moved the lorry and trailer to a secure location, where the bodies will be removed amid attempts to identify the victims.
Essex Police said it was called by the ambulance service shortly before 1.40am on Wednesday following the discovery of a lorry container with people inside at Waterglade Industrial Park, Eastern Avenue, Grays.
Some 39 people were pronounced dead at the scene.
Chief Superintendent Andrew Mariner said: "This is a tragic incident where a large number of people have lost their lives. Our enquiries 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 believe the lorry is from Bulgaria and entered the country at Holyhead on Saturday 19 October and we are working closely with our partners to investigate.
"We have arrested the lorry driver in connection with the incident who remains in police custody as our enquiries continue. I appreciate this cordon is going to disrupt the activity of local businesses in the area and we will work to ensure that disruption is kept as short as possible. We are working with Thurrock Council to mitigate against any impact our investigation scene will have locally."
The man remains in police custody in Essex.