A missing Vietnamese woman who texted her mother to say “I can’t breathe” is feared to be among 39 people found dead in a refrigerated trailer in Essex.

Pham Thi Tra My, 26, sent a message saying “I’m dying” at the time the lorry was being shipped from Belgium to the UK on Tuesday night. She has not been heard from since.

Detectives had initially believed all 31 men and eight women found in the trailer were Chinese, but on Friday night they said the nationality of those killed was “now a developing picture”.

Details of possible Vietnamese victims emerged as police arrested a third person on suspicion of people trafficking and manslaughter.

Ms Pham was known to have planned to travel to England, according to a human rights group in contact with her family. Her brother told the BBC they had paid £30,000 to people smugglers and her last known location had been Belgium.

Hoa Nghiem, a Hanoi-based activist with Human Rights Space, said the family was worried their daughter was among the victims as “the last dying text from her was coincidently in time”.

She shared a screenshot and a translation of the message, which read: “I’m sorry Mom. My path to abroad doesn’t succeed. Mom, I love you so much! I’m dying bcoz I can’t breath... I’m from Nghen, Can Loc, Ha Tinh, Vietnam... I am sorry, Mom”.

The text was sent at 4.28am on Wednesday Vietnamese time, the screenshot shows. In Britain the time would have been 10.28pm, when the trailer was in transit between Zebrugge and Purrfleet.

The bodies were found on an industrial estate in Grays just over three hours later.

An Essex Police spokeswoman told The Independent officers had received no information indicating whether Ms Pham was among the victims. In a statement issued on Friday night, deputy chief constable Pippa Mills said the force “will not be commenting on any speculation about the nationalities of those who have tragically lost their lives”.

39 bodies found in lorry container in Essex Show all 15 1 /15 39 bodies found in lorry container in Essex 39 bodies found in lorry container in Essex Thirty-nine bodies have been found in a lorry container in Essex, police have said PA 39 bodies found in lorry container in Essex The discovery of 38 adults and one teenager was made at an industrial estate in Thurrock Reuter TV 39 bodies found in lorry container in Essex Police said they believed the lorry had come from Bulgaria and entered the UK at Holyhead, in Wales, on Saturday. Getty 39 bodies found in lorry container in Essex Essex Police said it had launched a murder investigation after its officers were called to Waterglade Industrial Park, in Grays, in the early hours of Wednesday morning Reuter TV 39 bodies found in lorry container in Essex A 25-year-old-man from Northern Ireland has been arrested on suspicion of murder PA 39 bodies found in lorry container in Essex In a statement, Chief Superintendent Andrew Mariner said: “This is a tragic incident where a large number of people have lost their lives. Our enquiries are ongoing to establish what has happened. “We are in the process of identifying the victims, however I anticipate that this could be a lengthy process. Reuter TV 39 bodies found in lorry container in Essex THURROCK, ENGLAND - OCTOBER 23: Police stand near the site where 39 bodies were discovered in the back of a lorry on October 23, 2019 in Thurrock, England. The lorry was discovered early Wednesday morning in Waterglade Industrial Park on Eastern Avenue in the town of Grays. Authorities said they believed the lorry originated in Bulgaria and entered the country at Holyhead on October 19. The suspected driver was arrested in connection with the investigation. (Photo by Leon Neal/Getty Images) Leon Neal Getty 39 bodies found in lorry container in Essex THURROCK, ENGLAND - OCTOBER 23: A Police forensic investigation team are parked near the site where 39 bodies were discovered in the back of a lorry on October 23, 2019 in Thurrock, England. The lorry was discovered early Wednesday morning in Waterglade Industrial Park on Eastern Avenue in the town of Grays. Authorities said they believed the lorry originated in Bulgaria and entered the country at Holyhead on October 19. The suspected driver was arrested in connection with the investigation. (Photo by Leon Neal/Getty Images) Leon Neal Getty 39 bodies found in lorry container in Essex Police activity at the Waterglade Industrial Park in Grays, Essex, after 39 bodies were found inside a lorry container on the industrial estate. PA Photo. Picture date: Wednesday October 23, 2019. Early indications suggest there 38 are adults and one teenager, police said. The lorry is from Bulgaria and entered the country at Holyhead, North Wales, one of the main port for ferries from Ireland. See PA story POLICE Container. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA 39 bodies found in lorry container in Essex THURROCK, ENGLAND - OCTOBER 23: A Police forensic investigation team are parked near the site where 39 bodies were discovered in the back of a lorry on October 23, 2019 in Thurrock, England. The lorry was discovered early Wednesday morning in Waterglade Industrial Park on Eastern Avenue in the town of Grays. Authorities said they believed the lorry originated in Bulgaria and entered the country at Holyhead on October 19. The suspected driver was arrested in connection with the investigation. (Photo by Leon Neal/Getty Images) Leon Neal Getty 39 bodies found in lorry container in Essex Thurrock deaths: 39 bodies found in lorry container in Essex Reuter TV Reuter TV 39 bodies found in lorry container in Essex Police activity at the Waterglade Industrial Park in Grays, Essex, after 39 bodies were found inside a lorry container on the industrial estate. PA Photo. Picture date: Wednesday October 23, 2019. Early indications suggest there 38 are adults and one teenager, police said. The lorry is from Bulgaria and entered the country at Holyhead, North Wales, one of the main port for ferries from Ireland. See PA story POLICE Container. Photo credit should read: Aaron Chown/PA Wire Aaron Chown PA 39 bodies found in lorry container in Essex Police cordon at the road leading to the Waterglade Industrial Park in Grays, Essex, after 39 bodies were found inside a lorry container on the industrial estate. PA Photo. Picture date: Wednesday October 23, 2019. Early indications suggest there 38 are adults and one teenager, police said. The lorry is from Bulgaria and entered the country at Holyhead, North Wales, one of the main port for ferries from Ireland. See PA story POLICE Container. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA 39 bodies found in lorry container in Essex Police officers are seen at the scene where bodies were discovered in a lorry container, in Grays, Essex, Britain October 23, 2019. REUTERS/Hannah McKay HANNAH MCKAY Reuters 39 bodies found in lorry container in Essex Police officers are seen at the scene where bodies were discovered in a lorry container, in Grays, Essex, Britain October 23, 2019. REUTERS/Hannah McKay HANNAH MCKAY Reuters

She added: “We owe it to those who have died to get this investigation right and speculation is not helpful. It may in fact hinder our investigation and its progress.

“We gave an initial steer on Thursday on nationality, however this is now a developing picture. As such I will not be drawn on any further detail until formal identification processes approved by Her Majesty’s Coroner have taken place.”

The Vietnamese Embassy in London confirmed it had contacted Essex Police about missing people after receiving requests from worried families.

“The embassy has not yet received any official confirmation from the British relevant agencies,” a spokesperson said.

Ms Pham had travelled to China and planned to get to England through France, according to Ms Nghiem, who added: “Our contact is getting more alerts that there could be more Vietnamese people in the truck.”

Mimi Vu, an expert on trafficking of Vietnamese young people to Europe, said Ms Pham’s texts appeared to be authentic. She told The Guardian: “She writes her name and where she is from, which is very important, to tell people where she should be buried.”

She added people were sometimes given fake Chinese passports when they were trafficked through China.

VietHome, a news website for Vietnamese expatriates in Britain, said it had received information about 10 families with missing relatives feared.

The BBC said it knew of six Vietnamese nationals whose families fear they were among victims on the trailer. The brother of one 19-year-old woman told the broadcaster his sister had called him on Tuesday morning to say she was getting into a container in Belgium and was turning off her phone to avoid detection.

Smugglers have returned deposits paid by families of suspected victims, according to the BBC.

Some 702 Vietnamese people were referred to UK authorities as suspected victims of trafficking and modern slavery last year. The only foreign country with a higher number of victims in Britain was Albania.

Police investigating the Essex lorry deaths arrested a 48-year-old man from Northern Ireland at Stansted Airport on Friday on suspicion of manslaughter and conspiracy to traffic people.

It followed a couple from Warrington, Cheshire, being detained on suspicion of the same offences overnight on Thursday. Thomas Maher and his wife Joanna, both 38, were reported to be the last known owners of the Bulgarian-registered Scania lorry cab which picked up the refrigerated trailer. They claimed to have sold it to a company in Ireland 13 months ago.

The driver of the truck, 25-year-old Mo Robinson from Northern Ireland, remains in custody after being arrested on suspicion of murder on Wednesday.

Support free-thinking journalism and attend Independent events

A retired detective who investigated the Morecambe Bay cockling tragedy, in which 23 Chinese illegal migrant workers drowned in 2004, suggested the victims found in the Essex lorry could have been trafficked by a Snakehead gang.

“These are criminal travel agents really – you go to a Snakehead to say you want to be trafficked to an economic opportunity and usually you’ll borrow quite a significant amount of money,” Mike Gradwell, a former Lancashire Police detective superintendent, told BBC Breakfast.

It is not yet known when the victims entered the sealed refrigerated trailer, in which temperatures can be as low as -25C.

Joachim Coens, chief executive of Zeebrugge port, said it was unlikely people were loaded into the container there, while mayor Dirk De Fauw, who is also the chairman of the port, claimed it was “virtually impossible” the victims climbed into the trailer at the Belgian border.

The trailer arrived at Purfleet’s port at around 12.30am on Wednesday, while the lorry cab came from Northern Ireland through Holyhead in North Wales on Sunday.

They left Purfleet shortly after 1.05am. Police were called to Waterglade Industrial Park on in Grays at 1.40am after the bodies were found. Detectives have not yet confirmed who raised the alarm after finding the victims.