Essex police Deputy Chief Constable Pippa Mills read a statement following the discovery of 39 bodies in a lorry container on Wednesday.
All 39 people died, according to the statement. Early indications are that one was a teenager and the rest were adults.
The murder suspect, a 25-year-old male from Northern Ireland, remains in custody.
Police were called to Waterglade Industrial Park in Grays, Essex, by the ambulance service in the early hours of the morning. The force said the lorry travelled from Bulgaria and entered the UK via Holyhead in north Wales.
Police are working on identifying the bodies as a “priority” and warned the investigation could be a “lengthy process”. Mills appealed for anyone with information to come forward.