Police say Ronan Hughes and his brother Christopher are crucial to their inquiries into the discovery of the bodies of 31 men and eight women in the container on an industrial estate in Grays, to the east of the British capital.
The brothers are wanted on suspicion of manslaughter and human trafficking and Daniel Stoten, the officer leading the police inquiry, said they had spoken by telephone to Ronan Hughes recently but they needed to question them in person.
A Northern Irish man is facing multiple counts of manslaughter charges in relation to the discovery of 39 bodies in a truck near London last week, Irish broadcaster RTE said on Friday.
Eamon Harrison, 23, appeared in court in Dublin after the endorsement of a European Arrest Warrant, RTE said
Police originally said the victims were believed to be Chinese but Vietnam fears many of the deceased were its citizens.