Authorities believe the truck, which originated in Bulgaria, entered the UK through the Welsh port of Holyhead over the weekend. A regular ferry service connects Holyhead with the Irish capital, Dublin.
"We are in the process of identifying the victims, however I anticipate that this could be a lengthy process," Chief Superintendent Andrew Mariner said in a statement.
"We have arrested the driver in connection with the incident who remains in custody as our inquiry continue" Mariner added.
Police have placed a cordon around the area and say they are working with the local council "to mitigate against any impact our investigation scene wants to have locally."
The UK Home Secretary, Priti Patel, said she was "shocked and saddened" by what she called the "utterly tragic incident," according to her Twitter account.
This is a developing story.