Detectives are continuing to question two people from Cheshire over the deaths of 39 migrants whose bodies were found inside a lorry in Essex.
The couple, named locally as haulage boss Thomas Maher and his wife Joanna, both 38, of Warrington, were held on suspicion of 39 counts of manslaughter and people trafficking.
A 48-year-old man from Northern Ireland was later detained at Stansted Airport on Friday on suspicion of conspiracy to traffic people and manslaughter, Essex Police said.
It followed the arrest of the driver of the Scania truck on suspicion of murder on Wednesday, who remains in custody.
On Thursday, detectives were granted more time to question Mo Robinson, 25, from Northern Ireland, after the bodies of eight women and 31 men were found in the refrigerated trailer in an industrial park in Grays in the early hours of Wednesday.
In Belgium, police are hunting the driver who delivered the trailer to Zeebrugge, the port it left before arriving in the UK.
Deputy Chief Constable Pippa Mills said she would give no more details about the nationalities of the victims until formal identification had taken place.
The trailer arrived at Purfleet at around 12.30am on Wednesday, and was picked up by the cab, known as the tractor, which arrived from Northern Ireland via Holyhead in North Wales on Sunday.
The lorry left the port at Purfleet shortly after 1.05am before police were called to the Waterglade Industrial Park on Eastern Avenue in Grays at 1.40am.
China has called for joint efforts to counter human smuggling, while vigils have been held in London and Belfast to pay tribute to the victims.
The Vietnamese embassy in London has said some families have contacted them asking about relatives but it is yet to receive any information from police.