The driver, a 25-year-old man, had been arrested.
British police said they had arrested a man on suspicion of murder after the bodies of 39 people were found on Wednesday in a truck container at an industrial estate to the east of London.
Police said the truck was believed to have come from Bulgaria and to have entered Britain at Holyhead in Wales on Saturday. The driver, a 25-year-old man from Northern Ireland, had been arrested, they said.
“We are in the process of identifying the victims, however I anticipate that this could be a lengthy process,” said Chief Superintendent Andrew Mariner.