A family in Vietnam fears that one of the 39 people found dead in a refrigerated trailer in Essex this week was their daughter.

Releasing the devastating last text message from Pham Thi Tra My, her family said they were desperately seeking information and trying to verify whether she was one of those who had died.

Initial reports by police had suggested all the victims were believed to be Chinese.

Essex police declined to comment when approached by the Guardian. In a separate statement on Friday, the force said: “As our investigations continue, the picture may change regarding identification.” Meanwhile, two more people have been arrested on suspicion of manslaughter.

While the police have not yet confirmed the identities of those in the trailer, the family’s fears highlight the difficulties in obtaining information on trafficked individuals.

In the message to her mother, feared to have been sent in her final moments, the 26-year-old wrote that she was struggling to breathe and apologised.

The message, translated into English by a family acquaintance, was received at 4.28am on Wednesday morning in Vietnam, a screenshot shows. This would have been 10.28pm UK time, when the trailer carrying the victims would have been in transit – and just four hours before the ambulance was called and the bodies were discovered.

In messages from an account nicknamed ‘daughter’, Pham wrote: “I’m sorry Mum. My journey abroad hasn’t succeeded. Mum, I love you so much! I’m dying because I can’t breathe … I’m from Nghen, Can Loc, Ha Tinh, Vietnam … I am sorry, Mum.” One message has been blanked out because it mentioned the family’s address.

The screenshot also shows that Pham has not accessed the account since Wednesday.

The Vietnamese embassy in London confirmed it had contacted police in regard to a missing woman feared to be one of the 39 people found dead.

A spokesman for the embassy said it had been contacted by a family in Vietnam who claimed their daughter had been missing since the lorry was found.

The spokesman said: “We have contacted Essex police and we are waiting for an answer.”

The BBC later reported that Pham’s family had to pay £30,000 to traffickers for her journey to the UK. A family acquaintance confirmed to the Guardian that Pham’s family had to remortgage their home to pay the sum.

It was also reported two other Vietnamese families have relatives missing. A 26-year-old man and a 19-year-old woman.

The 19-year-old’s brother told the BBC she called him early on Tuesday to say she was getting into a container and would be switching her phone off to avoid being caught. They had not heard from her since and the people smugglers had since returned money to the family. Relatives of the 26-year-old, with whom she was travelling with, had also been given some money back.

Trafficked migrants are rarely on any kind of formal manifesto and victims’ families may in some cases be in danger themselves if they acknowledge their disappearance. They may also fear that by going public they will put their loved one at risk of arrest if they are in fact still alive.

Mimi Vu, a leading expert on trafficking of Vietnamese young people to Europe and the UK, said the texts appeared to be authentic. “She writes her name and where she is from, which is very important, to tell people where she should be buried. It is important for the body to be returned to the place of birth,” said Vu, who has been working in Vietnam on trafficking and slavery issues for the past seven years.

Vu noted that occasionally people were given false Chinese passports, if they were trafficked through China, which could lead to confusion.

Following Pham’s disappearance, her brother posted a message on a forum regularly used by Vietnamese people looking for missing relatives.

He wrote that Pham, who lived in the Nghen township in Can Loc district, left the rural province of Ha Tinh in northern Vietnam on 3 October before travelling to the capital, Hanoi, to “finalise her papers to go China”.

She then flew to France and on to the UK, he said. However, she was allegedly arrested on arrival in the UK before being sent back to France.

In the post, her brother added: “She was arrested a few days ago by UK police and they returned her to France. Now we heard that she might have died. I am posting to ask for her whereabout and if you tell me your projection about her situation.”

Hoa Nghiem, a coordinator at Human Rights Space, a civic network based in Vietnam, said she had spoken with a lawyer who was trying to help the family.

She said: “The girl’s family got in touch with the lawyer – who has dealt with cases like this before – asking if she could help. They are desperate for any information about their daughter.

“They just want any updates and have been posting on forums and speaking to the lawyer about getting help.”

Nghiem said she knew of at least six Vietnamese families who were concerned that their relatives were among the 39 people found dead in the refrigerated container.

“They lost contact with their family members who were travelling to the UK through China. They were worried if their family members could be in the truck,” she added.

Three of the families, all with missing sons – one born in 2004 – had been sent word that their loved ones would shortly arrive in Britain. “They said they received information that their family members were about to reach the UK on October 23, that they should be prepared to pay. But then the contact disappeared,” Nghiem said.

The BBC reported that another 26-year-old woman who was smuggled via a similar route from Vietnam through China and onto Belgium had been forced to switch off her phone while in the container to avoid capture and her family had not heard from her since. The traffickers who arranged her journey had since returned some of the money back to her family.

The province of Ha Tinh is a hotspot for trafficking, with many immigrants travelling into China first obtaining a Chinese passport and then travelling on to Europe.

“I understand that we are getting to know more and more about similar situations. The province is known for sending people illegally to Britain. When they get to China they then travel on from there,” added Nghiem.

Vietnam was the country with the third highest number of suspected victims of modern slavery in the UK in 2018, as identified by the national referral mechanism (NRM), the official framework for identifying victims of human trafficking and ensuring they receive the appropriate protection and support.

There were 702 Vietnamese modern slavery victims referred last year, behind 947 Albanians and 1,625 UK nationals.

A 2012 government report assessing human trafficking routes highlighted a journey from Vietnam to UK that passed through China, Russia and northern or western Europe.