A truck in which 39 bodies were discovered is driven from the site to a secure location where further forensic investigation can take place, October 23, 2019 in Grays, Essex, England. Getty

London — British police arrested two more suspects Friday in connection with the deaths of 39 people found in the back of a container truck in southeast England as the investigation into one of the country's worst human smuggling cases geared up. Police said the man and the woman, both 38 and from Warrington, a town in northwest England, were arrested on suspicion of manslaughter and conspiracy to traffic people. The 25-year-old driver of the truck remains in custody on suspicion of murder.

The new arrests came as police began the grim process of conducting post-mortem examinations of the dead. The remains of 11 people from the truck were transported by ambulance from the Port of Tilbury to a mortuary under police escort on Thursday, and the post-mortems were to begin Friday.

Essex Police said 31 men and eight women were found dead in the truck early Wednesday at an industrial park in Grays, a town about 25 miles east of London.

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Possible Vietnamese victims

Although U.K. police said they believed the dead were Chinese citizens, Chinese officials told reporters in Beijing that the nationalities and identities of the victims had not yet been confirmed, and there were Vietnamese families have spoken up with fears their relatives could be among the dead.

CBS News' partner network BBC News spoke to the families of three Vietnamese people who said their relatives, missing for days, could have been in the trailer. The family of one woman told the BBC she sent a text saying she was suffocating.

The family told the BBC that Pham Tra My, 26, sent a message on Tuesday night but that they had been unable to contact her since. The family told BBC and other British media they paid almost $50,000 to get her smuggled into Britain.

The other two Vietnamese families that contacted the BBC said they had not heard from their relatives, a 26-year-old man and a 19-year-old woman.

A truck in which police found the bodies of 39 people is seen parked on the Waterglade industrial park in Grays, Essex, southeast England, on October 23, 2019. BBC

Britain's Daily Mail newspaper said My's family reported receiving a last text from her late Tuesday night, as the shipping container carrying the smuggling victims is believed to have been in transit from Belgium to the U.K.

According to the newspaper, the text read: "I'm sorry Mum. My journey abroad hasn't succeeded. Mum, I love you so much. I'm dying because I can't breathe."

China helping with probe

Chinese Foreign Ministry spokeswoman Hua Chunying said China was working in cooperation with local British authorities.

"No matter where these victims come from, this is a great tragedy which drew the attention of the international community to the issue of illegal immigration," she said. "The international community should further strengthen cooperation in this area, strengthen sharing of information and intelligence ... to prevent such tragedies from happening again."

Hua said Chinese authorities were also seeking information from police in Belgium, since the shipping container in which the bodies were found was sent to England from the Belgian port of Zeebrugge.

Human smuggling from China is believed to have fallen drastically in recent years amid the country's rapidly growing domestic economy. However, some Chinese, particularly those with lesser education, continue to be drawn to Europe and North America by the promise of much higher wages than they can earn at home, despite the considerable risks involved.

Parts of China, especially the southeastern province of Fujian, have long histories of sending migrants abroad.

The issue is a difficult one for China's ruling Communist Party, which is intensely sensitive about China's international image and has staked much of its legitimacy to rule on improving living standards for the bulk of China's 1.4 billion people.

A convoluted, deadly journey

British police believe the truck and container took separate journeys before ending up at the industrial park in Grays. They say the container traveled by ferry from Zeebrugge to Purfleet, England, where it arrived early Wednesday and was picked up by the truck driver and driven the few miles to Grays.

Map showing locations key to investigation into discovery of 39 bodies in the back of a truck near Purfleet, England. Google Maps/BBC

The truck cab, which is registered in Bulgaria to a company owned by an Irish woman, is believed to have traveled from Northern Ireland to Dublin, where it caught a ferry to Wales, then drove across Britain to pick up the container.

Global Trailer Rentals Ltd told Ireland's national broadcaster RTE the trailer it owns was leased Oct. 15 in County Monaghan, in Ireland, at a rate of 275 euros ($299) per week. The Dublin-based company said it will make the data from its tracking system available to investigators.

The company's directors told RTE it was "shell-shocked" at the news.

Risking it all to reach the U.K.

Groups of migrants have repeatedly landed on English shores using small boats to make the risky Channel crossing, and migrants are sometimes found in the back of cars and trucks that disembark from the massive ferries that link France and England.

But Wednesday's macabre find in an industrial park was a reminder that criminal gangs are still profiting from large-scale trafficking.

The tragedy recalls the deaths of 58 Chinese migrants who suffocated in a truck in Dover, England, in 2000 after a perilous, months-long journey from China's southern Fujian province. They were found stowed with a cargo of tomatoes after a ferry ride from Zeebrugge, the same Belgian port that featured in the latest tragedy.