LONDON — Three more people were arrested on suspicion of human trafficking, British police said Friday, as part of a sprawling investigation into how the bodies of 39 people came to be found in a truck container in southeastern England.

A man and a woman, both 38 and from Warrington in Cheshire, northwest England, were arrested on suspicion of conspiracy to traffic people and on 39 counts of manslaughter, Essex police said in a statement Friday.

In addition, a 48-year-old man from Northern Ireland was arrested at Stansted Airport in England on suspicion of manslaughter and conspiracy to traffic people.

"This is a fast-moving investigation involving significant police resources dedicated to finding out the truth of what happened to the 39 people found dead," Essex Police Deputy Chief Constable Pippa Mills told reporters.

British police typically do not name suspects until or unless they are charged and police urged the media and the public not to speculate over their identities.

The latest arrests come after a 25-year-old man from Northern Ireland, who police say was driving the truck, was arrested Wednesday on suspicion of murder after the bodies were discovered. He remains in custody.

The trailer he was allegedly driving had arrived at the docks in Essex, southern England, having traveled from Zeebrugge in Belgium. The bodies were found in the town of Grays, east of London, more than an hour later at 1:40 a.m. Wednesday, officials said.

On Thursday, authorities began the slow process of determining the cause of the deaths and identifying the bodies. At least 11 were taken to Broomfield Hospital in Chelmsford, and the local coroner began postmortem examinations Friday. Police warned that the process could be "lengthy."

"This is all being done in close liaison with HM Coroner and we continue to work to ensure that the dignity of victims and respect for their loved ones are at the forefront of our investigation," the police statement said.

Initially Thursday, police identified the dead as Chinese nationals, and the story gained intense coverage in China.

"We read with heavy heart the reports about the death of 39 people in Essex, England," China's Embassy in London said in a statement to NBC News.

But Friday, Mills declined to comment further about the nationalities of those who died until a formal identification process is completed.

The Embassy of Vietnam in London said it had received requests from families in Vietnam asking them to help verify whether their loved one were among the victims. Embassy officials said they received no confirmation from British authorities as of Friday.