LONDON — The police in Vietnam arrested two people on Friday as part of the investigation into the deaths of 39 people whose bodies were found in a refrigerated tractor-trailer in southeastern England, last week.
The arrests were made on the same day that an official from the Essex Police, which are conducting the main investigation into the case, urged two brothers from Northern Ireland to turn themselves in for questioning in the case.
The developments came as officials in several countries, including Belgium, Britain, China, Ireland and Vietnam, were still trying to identify the victims found in Essex and to trace their path in a case with all the hallmarks of a human smuggling operation.
A 23-year-old truck driver from Northern Ireland, Eamon Harrison, was also charged in Ireland on Friday with 39 counts of manslaughter, as well as human trafficking and immigration offenses, in connection to the case.
Mr. Harrison was detained last week on unrelated charges but was thought to be a person of interest in the truck deaths. The Irish authorities said on Friday that they had begun the extradition process to send Mr. Harrison to Britain.
He is the second truck driver to be charged in the case. Maurice Robinson, 25, also of Northern Ireland, was charged on Saturday with 39 counts of manslaughter and a conspiracy to traffic people.
Mr. Robinson made his first appearance in court on Monday, where a judge ordered him held in custody.
Although little is known about the victims or how they died, the discovery of the bodies sheds a grim light on global human smuggling networks and the dangerous journeys that many undertake to reach European countries in search of jobs and a better future.
The refrigerated container was found last week in an industrial park a few miles away from the ferry terminal at Purfleet, England, where the container had arrived by ship from Belgium.
The Essex police said they initially believed the victims were from China, but grieving families in Northern Vietnam have said they feared that missing relatives were among the victims. The Vietnamese police began a criminal investigation after 10 families from Ha Thinh Province reported the disappearance of their relatives.
Two other men and a woman were detained in the investigation by the British police and later released on bail. And although the connection among the suspects remains unclear, the case increasingly seems to have some roots in the border between Northern Ireland and the Republic of Ireland.
On Friday, Daniel Stoten, detective chief inspector with the Essex Police, urged two brothers from County Monaghan, just south of the Irish border, to turn themselves in to the police in Northern Ireland for questioning.
Mr. Stoten told reporters that Ronan Hughes, 40, and his brother Christopher, 34, are wanted on suspicion of manslaughter and human trafficking.
“Ronan and Christopher, hand yourselves into the Police Service of Northern Ireland,” he said at a news briefing on Friday. “We need you both to come forward and assist us with this investigation.”
In a startling twist, Mr. Stoten said that detectives had spoken to Ronan Hughes by phone, but added that the police wanted to have a conversation with both men in person. It was not immediately clear what was said during the phone conversation.
Countries like Belgium, France and Britain have increased border security checks at some ports in recent years to prevent illegal smuggling, leading migrants to take greater risks when attempting to reach their destinations.
On Wednesday, 12 men were found alive in the container of a truck in Belgium, a week after the deadly discovery in Essex. The driver, who was transporting a shipment of fruit and vegetables, called the police after he suspected that people were in the back of the vehicle.
The police found 11 men from Syria and one from Sudan in the truck. The driver was not charged. On Sunday, eight people, including two children, were found in a refrigerated truck at the ferry terminal in Calais, in northern France, according to local news outlets.
Ed O’Loughlin contributed reporting from Dublin.