Police are continuing to question four suspects over the deaths of 39 people whose bodies were found inside a lorry in Essex.
A man was detained at Stansted Airport on Friday on suspicion of manslaughter and conspiracy to traffic people.
A couple from Warrington were also held on the same charges, while the lorry driver has been held on suspicion of murder since Wednesday.
The arrests came as efforts to remove and identify those who died continued.
Police officers have been seen at the home address of the arrested couple, Joanna and Thomas Maher.
Before their arrest, the 38-year-olds denied having anything to do with the lorry and told reporters they sold it last year.
The man arrested at Stansted Airport is aged 48 and is from Northern Ireland.
Police have been given extra time to question the 25-year-old lorry driver, Mo Robinson, of County Armagh.
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The bodies of eight women and 31 men were found in the refrigeration trailer at an industrial park in Grays.
The trailer arrived in Purfleet on the River Thames from Zeebrugge in Belgium at 00:30 BST on Wednesday.
It left the port shortly after 01:05 the same day and the bodies were discovered at Waterglade Industrial Park about 35 minutes later.
Police in Belgium are trying to trace the route of the trailer and the driver who delivered it to Zeebrugge.
“We would like people to be arrested as soon as possible,” said a spokesman for the Belgian prosecutor’s office.