LONDON -- A British man admitted on Monday plotting to assist unlawful immigration after 39 Vietnamese migrants were found dead in the back of a truck he was driving.

The driver, Maurice Robinson, also pleaded guilty to acquiring criminal property but was not asked to enter a plea on 41 other charges including 39 counts of manslaughter in a case which has shone a light on the illicit human smuggling trade.

Robinson, 25, from Northern Ireland, was addressing a London court hearing from prison a month after the 39 bodies were found in the truck on an industrial estate in Grays, about 20 miles (32 km) east of London.

Thirty-one of the victims were men or boys, and eight were women. The oldest was 44 and three were aged under 18, including two 15-year-old boys. Most were from Nghe An and Ha Tinh provinces in north-central Vietnam, where poor job prospects and other factors fuel migration.

Many impoverished people from Asia, the Middle East and Africa make perilous journeys to Western Europe in hopes of a better life, often after paying huge sums to criminal gangs.

Prosecutor William Emlyn Jones told Monday’s hearing at London’s Old Bailey criminal court that it was likely to be a large and complex case.

Robinson admitted to having conspired with others between May 1, 2018, and Oct. 24 this year to commit an offence of assisting unlawful immigration, and to acquiring cash which he knew or suspected came from criminal conduct.

No trial date was set, and Robinson is next due in court on Dec. 13.

Another man from Northern Ireland, 23-year-old Christopher Kennedy, appeared at a separate hearing on Monday, charged with conspiring to arrange the travel of people with a view to their exploitation, and conspiracy to break immigration laws.

His case was also adjourned to the Old Bailey until Dec. 13.

A third suspect, Eamon Harrison, 22, also from Northern Ireland, has also been charged with 39 counts of manslaughter and human trafficking and immigration offences. He was arrested in the Irish Republic and the British authorities have started proceedings to extradite him.

Police in Vietnam have arrested 10 people in connection with the deaths.

British police said late on Monday it made a further arrest in its probe.

A 36-year-old man was arrested in East London on “suspicion of manslaughter, conspiracy to traffic people and conspiracy to assist unlawful immigration” in connection with the death of the Vietnamese migrants, Essex Police said, adding he remains in custody for further questioning.