LONDON (AP) — Provisional postmortem examinations of 39 bodies found inside a shipping container in southeast England concluded the victims from Vietnam died of a combination of a lack of oxygen and overheating in a closed space, police said Tuesday.

The bodies were found Oct. 23 in the English town of Grays, east of London. Police said the people who died ranged in age from 15 to 44. The 31 men and eight women are believed to have paid people traffickers for their clandestine transit into England.

“Our teams are continuing to progress hundreds of lines of enquiry and are working with the National Crime Agency and other law enforcement agencies from across the globe to further their lengthy and complex investigation,’’ Essex county police said in a statement.

The revelations came as police made strides to identity suspects in the network believed to be responsible for the smuggling operation.

Gheorghe Nica, of Basildon in eastern England, was detained at Germany’s Frankfurt Airport on Jan. 29. The 43-year-old appeared at Chelmsford Magistrates’ Court on Saturday. He faces 39 counts of manslaughter and one count of conspiracy to assist unlawful immigration.

A second man, who is 22, was arrested in Northern Ireland Sunday on suspicion of manslaughter and facilitating unlawful immigration. He remains in police custody in southeast England but has not been charged or named yet.

Another suspect from Northern Ireland, Eamonn Harrison, 23, is being held in Ireland on charges of manslaughter and conspiracy to traffic people. He has a Dublin High Court hearing scheduled for Wednesday.

Prosecutors allege that Harrison drove the container to the port of Zeebrugge in Belgium, where it was put on a ferry to England and picked up at the other end by Maurice Robinson Robinson, who also is from Northern Ireland and has been charged with manslaughter.

Harrison’s lawyers have argued that Britain should not be able to seek his extradition from Ireland because the alleged offenses took place in Belgium.