December 29, 2025
HANOI – For months, gory details swirled in cyberspace about a shocking crime in Vietnam that began with an online hook-up and ended in bloody murder.
In late November, Doan Van Sang, then deputy head of Market Management Team No. 4 in Lang Son province – a mountainous area in the north-east near the border with China, about 160km from capital Hanoi – was arrested by the police. On Dec 3, he was formally charged with murder and detained without bail pending further investigation.
The killing of Mr Nguyen Xuan Dat on Jan 25 gripped netizens for months with its gruesome nature as amateur web sleuths attempted to solve the mystery, going online with their findings.
This case in 2025 was unique in that highly graphic information and evidence circulated online for weeks before the police made an arrest.
As the facts stand, Sang, 57, first “met” the victim through social media around 2020, where they reportedly engaged in “distorted, deviant” conversations involving sexually morbid or perverse topics. In January 2025, Sang called Mr Dat to his workplace, where the murder took place. According to the authorities, Sang made significant efforts to clean the crime scene, destroy evidence and dispose of the body.
The 36-year-old victim, Mr Dat, had reportedly been away from his family home in northern Thai Binh – now Hung Yen – province for about 10 years, working as a labourer and fishmonger before his death.
Online reports and unofficial documents widely circulated during the investigation suggest Mr Dat had been active in extreme forums under aliases like “datbeheading”, where he had expressed a “death wish” and fantasies that included being beheaded and cannibalised. While rumours of cannibalism were widespread on social media, official law enforcement and local news reports do not corroborate these claims.
Following Mr Dat’s disappearance, video clips and digital images of his final moments were uploaded onto the internet from February. For a fee, these were available on the dark web and Telegram channels, and on Chinese online community platform Baidu Tieba, which has forums catering to niche interests.
Several videos of a similarly disturbing nature, featuring a masked Vietnamese butcher, were also posted online in late July. It is unclear, at this point, who posted all this content, which the Vietnamese authorities have since attempted to scrub off the internet.
The explicit online content spurred Vietnam’s web sleuths to track down the killer and the victim, especially after it was determined that both were Vietnamese, based on the language spoken and the type of room layout unique to that region in the country, among other things.
The web sleuths combed through every social media platform available, including Facebook, Instagram, TikTok, Telegram and X – searching for clues, and speaking to people who might know something about the pair.
Doan Van Sang was arrested by the police and formally charged with murder on Dec 3. PHOTO: VIETNAM’S MINISTRY OF PUBLIC SECURITY/THE STRAITS TIMES
The rise of online ‘citizen detectives’
In an era where information flows more freely than ever and technology empowers individuals like never before, a new breed of crime solvers has emerged: the “citizen detective” or online amateur sleuth, armed with internet connection, an enthusiasm for true crimes and an insatiable curiosity, reshaping the landscape of criminal investigations – to the consternation or grudging acceptance of the authorities.
The concept of citizen involvement in crime-solving is not novel. Throughout the ages, communities have come together to protect themselves and solve local mysteries.
However, the internet has vastly expanded the depth and breadth of citizen detective work. What was once limited by geography and access to information has become globally practicable via a few clicks. With the rise of social media and online sites in the 21st century, internet users can easily log in to find and share information, working together with diverse skill sets to tackle ongoing criminal cases.
The digital sleuthing phenomenon is evident in Vietnam, too.
Enthusiastic online sleuths in Vietnam include a YouTuber with the local moniker Ke To Mo (A Curious Person), who has been active since 2017. Ke To Mo has over 21,000 followers on Facebook and 87,000 subscribers on YouTube. One of his videos on the Sang-Dat murder mystery chalked up nearly half a million views on YouTube.
Also active is a public Facebook group called The Investigation Group of Nguyen Xuan Dat Case, which was set up in August, and later renamed OSINT & Cyber Investigation Viet Nam. It has 1,800 members currently.
OSINT, short for open-source intelligence, is the practice of gathering and analysing information from publicly available sources such as social media, news, public records and websites, and using this data to identify trends, uncover insights or support investigations.
Ke To Mo posted the first episode of the investigation into the death of Mr Dat on YouTube in early September. Over three months, eight episodes and four special segments were featured, ranging from 10 minutes to half an hour each – some were only for viewers with paid membership. At the end of the series, Ke To Mo concluded that the killer was someone with “a high-profile and wholesome cover, but inside was a bloodthirsty devil”.
The victim’s identity was not too difficult to uncover, as Mr Dat was active on kink e-forums and his face was clearly visible in the unsettling videos and images uploaded online. But the quest to identify his killer took longer because the latter took significant precautions to hide his identity by donning a mask.
YouTuber Ke To Mo was among the first online sleuths attempting to solve the murder mystery. PHOTO: SCREENGRAB FROM KE TO MO/YOUTUBE/THE STRAITS TIMES
Viral document sparks interest, prompts action
Mr Dat was identified soon after an 88-page document, said to be the collective efforts of a group of internet sleuths, began circulating widely online in mid-November.
The online sleuths reportedly compiled a comprehensive file pointing to Sang as the prime suspect, using clues like his birthmark, a specific tattoo, and leaked conversations from deviant online forums he shared with his victim. They examined every detail of the crime scene, from floor tiles and window frames to the colour of the walls and the brand of washing-up liquid.
The viral spread of the document including video footage with graphic details of the alleged crime generated massive public attention, speculation and outrage, prompting the authorities to officially announce a formal investigation and subsequent arrest in a matter of weeks.
Ke To Mo and OSINT & Cyber Investigation Viet Nam are believed to be among the group of online sleuths instrumental in collating the digital investigation efforts.
The authorities have since warned that while these groups uncovered important details, the downloading and sharing of the resulting 88-page document is considered a violation of information security laws, as it contains “barbaric” and “toxic” content that could create social panic and obstruct the investigation.
The police stressed that citizens must not assume the role of investigators or circulate unconfirmed reports, and that public speculation and defamation “can result in serious legal consequences”.
Sang, who was later dismissed from his position, has been charged with murder under Clause 1, Article 123 of the Penal Code. If convicted, he could face jail time ranging from 12 to 20 years, life imprisonment or the death penalty, depending on the severity and specific circumstances of the crime.
He was a senior government official in a market management team. These teams in Vietnam are a specialised government enforcement body under the Ministry of Industry and Trade, responsible for inspecting and controlling markets to ensure compliance with commercial laws – such as monitoring prices and inspecting goods during festive periods and after natural disasters.
“No one can escape the truth, not even those who think they can live in the dark forever,” intoned Ke To Mo in the final episode of the Sang-Dat series.
The YouTuber declined to comment on the matter when approached by The Straits Times, saying: “Sharing information at this sensitive time is against the government’s policy, which we are not willing to do.”
In late November, Doan Van Sang, then deputy head of Market Management Team No. 4 in Lang Son province, was arrested by the police. PHOTO: DOAN VAN SANG/FACEBOOK/THE STRAITS TIMES
‘Sick to my stomach’
An amateur sleuth with the OSINT & Cyber Investigation Viet Nam group, who did not want to be named as the case is now officially a police matter, told ST that he got involved in tracking the real-life murder mystery as he was moved by “curiosity” about Mr Dat’s fate.
“When I saw the video clips (of his killing), I felt sick to my stomach and wanted to throw up. The barbarity was shocking.
“This case could have become obsolete and forgotten, like thousands of other mysteries that have never been solved,” he added.
With help from the internet, the process of uncovering the truth in Vietnam is now much faster and happens far more often, according to local cybersecurity specialist Ngo Minh Hieu, 36, better known as Hieu PC online.
Vietnam’s state media have reported many cases in which, thanks to netizens’ quick reactions and shrewd investigations, missing people have been found and hit-and-run incidents exposed.
In one instance in November, an 18-year-old female student in north-central Nghe An province was found safe and unharmed after wandering around for three days. Details were scant on the reason she went missing, but it was not due to criminal activity.
Dr Dao Trung Hieu, a criminology expert, said the Vietnamese online community has shown “an increasing sense of civic responsibility”.
“Many instances of law violations, from school violence and child abuse to offensive behaviour in public, have been detected, documented and reported to authorities, thanks to the keen observation of citizens,” he posted on Facebook on Nov 30.
Dr Hieu recalled one occasion when, after posting photos of two suspects who had allegedly murdered and robbed a motorbike taxi driver on his Facebook page, he received several leads on the perpetrators the very next day.
“Modern society relies not only on law enforcement but also on community participation to ensure order and safety,” he wrote. “When citizens speak out about wrongdoing, community attention creates pressure to hold those who commit wrongdoing accountable.”
But he warned against “turning this positive force into a tool for causing public panic”.
While it is true that open-source investigations and the rise of citizen-sleuthing can provide a measure of support for law enforcement agencies in some instances, the line between being a vigilante and breaking the law is a thin one, noted lawyer Tran Dai Lam at Hanoi-based firm ANVI.
The advent of “self-proclaimed detectives” and “online investigators” is inevitable in the digital age, he said.
“However, when citizens independently track leads, collect data and post it online, their actions do not constitute lawful investigative activity as they lack the mandate, tools and training required to gather evidence properly.”
This makes them susceptible to subjective speculation, possibly revealing investigative secrets, leaking evidence or inadvertently hindering investigations, he explained.
Vigilante investigators, even those acting with the best of intentions, should be discouraged, Mr Lam told ST. The most appropriate approach remains “providing information to the competent authorities for verification in accordance with legal standards”.
Still, cyber security specialist Mr Hieu, 36, acknowledged that it would be “difficult… to try to completely ban citizen investigations”.
“What is more important is to provide proper guidance and clearly defined boundaries so that the online community can become ‘eyes and ears’ that provide support for the authorities, rather than creating counterproductive risks that could jeopardise criminal investigations,” he added.
Vietnam has a low homicide rate of around 1.3 to 1.5 per 100,000 people, but reports in recent years suggest a rise in absolute murder numbers, with domestic issues, financial disputes and drug use as common drivers. However, reliable official solve rates are hard to find due to state secrecy around crime data, especially capital punishment, though police presence is strong and violent crime against tourists is rare.