A UK court on Monday remanded in custody a truck driver over the deaths of 39 Asian migrants he had been smuggling, in a case that has horrified Britain and sparked a search for their country of origin.Maurice Robinson, a 25-year-old from Northern Ireland known as Mo, was charged on Saturday with 39 counts of manslaughter, money laundering and conspiracy to assist unlawful immigration.As bereaved families in Vietnam prayed for their missing sons and daughters, some of whose identities emerged online, Robinson made no statement in a brief video appearance at a court in Chelmsford, northeast of London.He has been scheduled to enter a plea at London's Old Bailey court on November 25.The eight women and 31 men found on Wednesday in a refrigerated container in Essex, southeast England, were originally identified as Chinese. But several Vietnamese families have come forward since, saying they feared their relatives were among the dead, and UK authorities have walked back their original statement.Mourning in VietnamThe grim case has again cast light on the extreme dangers facing illegal migrants seeking better lives in Europe, and reopened debates across Britain about ways to prevent such tragedies from happening again.Britain is now conducting its largest murder investigation since the July 7, 2005 London suicide bombings that killed 52 people.Officials started collecting DNA samples from families in Nghe An and Ha Tinh, impoverished provinces in central Vietnam where most of the suspected victims came from. On Monday, Vietnam said Britain had sent documents to help with the complicated task of identifying the bodies, many of whom were believed to be carrying falsified passports. "The UK side has sent four sets of dossiers related to the Essex lorry deaths... for verification coordination," Vietnam's Deputy Foreign Minister Bui Thanh Son said. Vietnam's foreign ministry is gathering information on the suspected victims, the report said, after hair and blood samples were collected from several families. UK authorities arrested another man wanted in the case on Saturday in Dublin, while three others detained earlier have been released on bail.The man arrested Saturday is also believed to be in his 20s and from Northern Ireland, although no details about his identity have been released.UK Prime Minister Boris Johnson, who campaigned for stronger borders while pushing Britain to leave the EU in the 2016 referendum, signed a book of condolence Monday and laid flowers in Grays, east of London, where the truck was found.Interior minister Priti Patel was also due to answer questions about the case in parliament.The tragedy has plunged communities in central Vietnam into mourning as families desperately wait for news from their missing relatives. Vietnamese media reported that as many as 24 of the victims could be Vietnamese although officials have not confirmed the number. Central Vietnam has long been a source of illegal migration to Britain for people seeking better lives. Vietnamese migrants often work illegally in nail bars or cannabis farms, heavily indebted and vulnerable to exploitations.