Although little is known about the victims or how they died, the discovery of the bodies shed 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.

[Update: U.K. police have identified all 39 people found dead in a truck in Essex.]

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 Tinh 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 area 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, were 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.”