Police investigating the deaths of 39 people, believed to be Chinese nationals, found in a truck near London said they had arrested two people on Friday on suspicion of human trafficking and continued to quiz the driver as a murder suspect.

As China called on Britain to seek “severe punishment” for those involved in the deaths, police said they had detained a man and a woman, both aged 38, in Warrington, northwest England, on suspicion of conspiracy to traffic people and of 39 counts of manslaughter.

The 25-year-old truck driver from Northern Ireland remains in custody after being arrested following the grim discovery of the bodies in the back of his refrigerated truck on an industrial estate near London in the early hours of Wednesday morning.

He has not been formally identified but a source familiar with the investigation named him as Mo Robinson from the Portadown area of the British province. Detectives will decide later whether to charge him with an offense, release him or ask a court for more time to quiz him.

Late on Thursday, British authorities moved 11 of the victims – 31 men and eight women – to a hospital mortuary from a secure location at docks near to the industrial estate in Grays about 20 miles (30 km) east of London where the bodies were found.