Distraught Vietnamese families were seeking information about their missing loved ones last night amid growing fears that some of the 39 people who died in a refrigerated lorry in Essex were from the country.

Text messages from one woman, 26-year-old Pham Thi Tra My, to her mother matched the time of the tragedy. They said: “I’m dying because I can’t breathe.” She has not been heard from since, a family representative told the Guardian.

Several other families said that their loved ones had travelled via China to Europe, aiming to reach the UK, and that they had also been out of contact since early Wednesday morning, when the victims’ bodies were found in the trailer after being ferried from Belgium to Britain.

Twenty-four hours after police had said that the victims were all believed to be Chinese, the heartbreaking Vietnamese accounts provided the first evidence of who may have been trapped in the trailer, contradicting earlier information. It is understood that some of the missing Vietnamese people may have been travelling using false Chinese passports.

On Friday Essex police backtracked on its initial claim, with the deputy chief constable Pippa Mills telling reporters: “We gave an initial steer on Thursday on nationality. However, this is now a developing picture.” She said she would give no further information until a coroner approved formal identification processes.

The news came on a day of rapid developments in the investigation, with three new arrests in England and details emerging of security processes at the Belgian port where the trailer began the final leg of its journey.

A man and a woman from Warrington, both 38, were arrested on suspicion of conspiracy to traffic people and 39 counts of manslaughter, following raids at addresses in Cheshire on Friday morning. A 48-year-old man from Northern Ireland was later arrested at Stansted airport on suspicion of the same offences. The 25-year-old driver of the lorry, Mo Robinson, was still in custody.

In Vietnam, families responded to press reports of the identification of the victims as Chinese by taking to social media and contacting the country’s embassy in the UK with requests for help. It is believed that there are at least six families who fear their loved ones could have died in the container.

One of the victims is believed to be Pham. Releasing her last text message, her family said they were desperately seeking information and trying to verify whether she was one of those who had died.

In the message to her mother, feared to have been sent in her final moments, Pham 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 titled “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. She had previously told her family not to call her because the people taking her to the UK had forbidden calls.

Meanwhile, the BBC reported that Nguyen Dinh Luong, 20, was also feared to be among the 39 victims.

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 23 October, that they should be prepared to pay. But then the contact disappeared,” Hoa Nghiem, a coordinator at Human Rights Space, a civic network based in Vietnam, said.

It was also reported that 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 detection. They have not heard from her since and the people smugglers have since returned money to the family. Relatives of the 26-year-old, with whom she was travelling, have also been given some money back.

The Vietnamese embassy in London confirmed it had contacted police to seek further information. A spokesman said it had been contacted by a family in Vietnam who claimed their daughter had been missing since the lorry was found.

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.

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.

Essex police urged anyone who thinks their friends or relatives may have been in the trailer to contact them – even individuals who may be “living illegally in this country”.

“Please come forward and speak to us without fear,” Mills said. “I can assure you that your information will be received in strictest confidence and no criminal action will be taken against you.”

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. She 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 whereabouts and if you tell me your projection about her situation.”

Nghiem 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.”