The first was to be questioned over the death of an immigrant and the illegal transportation of 34 others found inside a shipping container at Tilbury Docks, Essex, after being arrested in Northern Ireland yesterday.
The 34-year-old, from Limavady, was arrested just after midday at Banbridge on the A1 in County Down, police said.
He was held on suspicion of manslaughter and facilitating illegal entry into the UK.
Meet Singh Kapoor, 40, from Afghanistan, was found dead inside the container at Tilbury on Saturday. Other members of the 34-strong group, including 13 children, survived. They are all Afghan Sikhs from Kabul.
A separate investigation is under way after a lorry containing 15 people was discovered at a service station in Ilminster, near Taunton, by police working alongside immigration officials.
A German national has been arrested on suspicion of facilitating the unlawful entry of people into the UK in connection with that investigation.
The foreign nationals – who told police they were from Eritrea and Kashmir – included a 15-year-old boy.
An Avon and Somerset Police spokesman said: “We received several calls raising concerns about noises coming from the back of a lorry.
“Officers stopped the lorry at a service station at about 1pm.
“They were checked by paramedics at the scene, but there are no ongoing concerns over their health or wellbeing and they have been provided with medical support and supplies.
“The lorry, which originated in mainland Europe, has been seized.”
The discovery of the group at Tilbury has again highlighted the issue of human trafficking.
Nick Alston, the Essex police and crime commissioner, said: “This incident should highlight for us the evil of the abuse and exploitation of vulnerable people of all sorts and in many different circumstances.
“We can be sure that almost all cases of human trafficking involve exploitation by callous, organised criminal gangs.”
The group found in the container were screaming and banging after arriving at the Essex port from Belgium. It is thought they had travelled by lorry from Afghanistan before spending 18 hours inside the container as it travelled from Zeebrugge.
The survivors are now in the care of the Home Office after being interviewed by police and are in the process of applying for asylum in the UK.
A Home Office spokesman said: “We are continuing to provide expert support to the Essex Police investigation. This tragic incident is a reminder of the devastating human consequences of illegal migration.
“All 34 are now in the process of claiming asylum in the UK and we are providing accommodation and support to those who require it while their cases are considered.”
Any decision will not be announced publicly.
The Limavady man is currently in the custody of the Police Service of Northern Ireland, awaiting transportation to the mainland to be questioned by Essex Police.