Northern Irish lorry driver Maurice “Mo” Robinson is due to appear in court tomorrow charged with 39 counts of manslaughter, conspiracy to traffic people, conspiracy to assist unlawful immigration and money laundering.
Essex also confirmed that a man arrested in Dublin by Irish police over an unconnected matter on Saturday “is a person of interest” in their investigation.
A spokesman said: “We are liaising with the Garda via telephone as this man is currently held outside the jurisdiction of the law of England and Wales.”
Three other people arrested over the deaths of 39 migrants in a lorry in Essex have been released on bail.
Detectives are still trying to identify the victims - many of whom are now thought to be Vietnamese.
ITV News said more than 20 of the victims were from Vietnam.
All of the victims have since been moved from the vehicle in Tilbury Docks to Broomfield Hospital in Chelmsford for post-mortem examinations to be carried out.
Essex Police are now working on the largest mass fatality victim identification process in its history, having previously said all were from China.
Investigators will look at tattoos, scars and belongings, including jewellery and clothing, with each of the victims said to have had some kind of bag.
More than 500 exhibits have been collected, including mobile phones, which will be downloaded and interrogated for any messages that could give clues to the identity of the victims or how they came to be in the back of the trailer.
Detectives are investigating a “wider conspiracy” after claims surfaced that the lorry could have been part of a convoy of three carrying around 100 people.
The Vietnamese Embassy in London has started a hotline while the ambassador to the UK, Tran Ngoc An, spoke to British Home Secretary Priti Patel on Friday night before meeting investigators from the National Crime Agency and Essex Police.
Lorry driver Robinson is expected to appear before magistrates in Chelmsford on Monday.
Three other suspects have been released on bail. A man and woman, both 38, from Warrington were arrested in Cheshire on Friday and a 46-year-old man from Northern Ireland was arrested at Stansted Airport later the same day.
All three had been questioned on suspicion of manslaughter and conspiracy to traffic people.
The man and woman from Warrington were released on bail until November 11 while the Northern Ireland man has been bailed until November 13, police said.
In Belgium, police are hunting the driver who delivered the trailer to Zeebrugge, the port it left before arriving in the UK.
It is not yet known when the victims entered the trailer, where temperatures can be as low as minus 25C (minus 13F) if the fridge is activated, or the exact route it travelled.