Women who are groped on trains in East Asia face the further threat of their assault being filmed and uploaded for sale online. In a year-long investigation, the BBC World Service's investigative unit, BBC Eye, has gone undercover to unmask the men cashing in on sexual violence.
It was the morning rush hour in Tokyo. The train was packed and rocky.
Takako (not her real name) was on her way to school. The 15-year-old tried to hold on to a grab bar.
Suddenly, she felt a hand pressing on her behind. She thought someone had accidentally bumped into her.
But the hand started to grope her.
"That's when I finally realised - it was molestation," Takako recalls.
The hand quickly disappeared in the crowd. "I couldn't do anything about it." She arrived at school in tears that day.
That was her first time being sexually assaulted on public transport, but Takako was molested almost daily for more than a year on her commute. On countless nights, she went to bed crying. "I felt like there was no hope in my life," she says.
Many women like Takako are targeted in public by sexual predators. In some cases, they face another violation - the attack is filmed and the videos are sold online.
Most videos follow the same pattern - a man secretly films a woman from behind and follows her on to a train. Seconds later, he sexually abuses her. The men act discreetly, and their victims can seem totally unaware. These graphic videos are then listed on the websites for sale.
In a year-long investigation, we traced the men behind three websites which sell and produce thousands of these sexual assault videos.
An epidemic in Japan
Encountering sexual abuse almost daily, Takako found herself unable to speak up during the act due to fear and shame. But every night, she covered her mouth with a towel and repeatedly practised in front of the mirror how to call out a harasser: "This person is a 'Chikan'!"
"Chikan" is a Japanese term describing sexual assault in public, especially groping on public transport. It also describes the offenders themselves.
Chikan perpetrators typically take advantage of crowds, and the victims' fear of causing a scene. In Japan, speaking too directly and openly may be seen as rude.
Thousands of arrests are made every year for Chikan offences, but many more go undetected and unpunished. Saito Akiyoshi, mental health professional and author of a book about Chikan, says that only about 10% of victims report the crime.
The Japanese police encourage victims and eyewitnesses to speak up, but the crime is far from being eradicated. The problem is so widespread that even the UK and Canadian governments warn travellers to Japan about it.
Chikan has been normalised by its prominence in Japan's adult entertainment industry. One of the most popular types of pornography in the country - the Chikan genre - has spread to other Asian countries.
One Chinese-language website called DingBuZhu (which means "I can't hold it" in Chinese) immediately caught our attention.
It's a marketplace for Chikan videos, filmed secretly on mobile phones in crowded public places, such as trains and buses. They are shot across East Asia, including Japan, South Korea, Taiwan, Hong Kong and mainland China.
Some videos cost less than a dollar. The site even once allowed users to order tailor-made abuse videos.
We also found links on DingBuZhu to two other websites - Chihan and Jieshe - with the same type of content.
There is a Telegram group with 4,000 members who share tips on how to sexually abuse women.
One name kept coming up on the Chikan websites - "Uncle Qi".
He was hailed as the guru in this community. Dozens of abuse videos were labelled as his work. On Twitter, he put up teasers of the websites' videos to his 80,000 followers. But who was he?
The Telegram group we had been monitoring revealed a clue. One day, an admin claimed in a series of messages that he had abused a woman with Uncle Qi.
The messages were accompanied by photos of a woman standing on what appeared to be a metro platform.
Within hours, we found a match for the location - Ikebukuro station in Tokyo.
BBC Eye investigates websites selling thousands of videos of men sexually assaulting women on trains, buses and other crowded public places across East Asia.
Watch on BBC iPlayer now (UK only), or on BBC Three at 22:50 BST on Thursday 8 June
And there were more leads pointing us to Japan.
The websites listed a Paypal account receiving Japanese yen which was linked to a Gmail address. When we put the address through Google Contacts, the profile picture that came up was a young man with an elaborate hairstyle and theatrical makeup.
A reverse image search put a name to the face - Noctis Zang, a 30-year-old Chinese-born singer living in Tokyo. He was the frontman of a metal band called The Versus.
Noctis had a glamorous public image, but we soon found something hidden behind it.
In early 2022, The Versus' photographer had alleged on Chinese social media platform Weibo that Noctis built "porn websites" alongside another band member, Lupus Fu.
He had posted pictures of a notebook, which showed some accounting and video categories similar to those on the websites. The photographer had also posted a video which appeared to show Noctis's browsing history, with links to Chihan, Jieshe and the admin pages of DingBuZhu.
Could this rock singer be Uncle Qi?
Unmasking the admins
Posing as a music talent scout called Ian, our undercover journalist met Noctis at a fancy rooftop bar in Tokyo.
They first talked about music, but the chat soon moved on to the subject of sex. When Ian said his company used to make porn films, Noctis's eyes lit up.
The two met several more times, and they even celebrated Noctis's birthday together.
Noctis introduced Ian to his fellow band member Lupus Fu, whose name had been mentioned by The Versus' photographer. Lupus, also from China, was studying sociology in Japan.
Ian said his company planned to invest in porn sites and asked if they knew anything about this business.
Noctis confessed he had "some exposure" through a friend, "Maomi", who had created his own porn sites with "metro" content.
Ian then casually brought up DingBuZhu.
Lupus and Noctis both laughed: "That's Maomi's website!"
They revealed that the person behind the Chikan websites was a Chinese man in Tokyo nicknamed Maomi. They said Maomi was reclusive and paranoid.
Noctis and Lupus also admitted that they played admin roles for the websites.
They spelled out their business model.
"In China, sex is the most suppressed," Noctis said, "Some men are very perverted, they just want to see women getting…" Lupus finished the sentence: "screwed over."
Lupus said he was in charge of promoting abuse videos on Twitter. Noctis revealed that he had uploaded more than 5,000 videos on the websites, received payments for the business and taken 30% of revenue. The rest he had transferred to Maomi.
Lupus also said he could help connect Ian to Maomi.
The Chikan club
On a quiet back street in the red-light district of Yokohama, a storefront decorated like a metro station catches your eyes. A sign spells out its concept: "legal Chikan trains".
In this sex club, called Rush Hour, customers can pay to enjoy the Chikan experience legally.
Its manager Hasuda Shuhei welcomes us on board. "We let people do things that can't be done outside. That's why people come here."
Inside, a sickly-sweet smell of cleaning products permeates the air. Private rooms are decorated like train carriages and equipped with a sound system that plays train announcements. Even the club's membership cards look exactly like Japan's transportation cards.
"I think it's important for men to be able to pay to vent in place like this, so they don't commit rape and other forms of sexual assault," says Hasuda.
Mental health professional Saito says that the matter is not as straightforward as Hasuda claims. He says that most Chikan perpetrators are aroused by the idea of domination over and humiliation of their victims.
"They do not treat their victims as equals, but as objects."
It's an opinion that rings true with Takako.
After months of assaults, she fought back one day. As she felt a hand reach for her skirt in a packed train carriage, Takako shouted at the top of her lungs and grabbed the assaulter by his wrist.
Takako took the man to court, where he only got a suspended sentence, even though he had previously been caught for Chikan offences.
Disappointed by the outcome of her case, Takako went on to start an anti-Chikan campaign, producing colourful badges reading "Chikan is a crime!" People can wear them to show they will not keep silent.
"It's a deterrent for criminals," says Takako, who is now 24. There is now an annual anti-Chikan badge design contest among Japanese high school students.
Maomi means "kitty cat" in Chinese. However, Lupus said his personality was more like a hamster. "He's harmless, but cautious of everything and he sometimes overreacts."
Lupus was right. Maomi repeatedly refused to meet Ian.
But on Chinese New Year's Eve, Ian's luck changed. Maomi agreed to a meeting at a karaoke bar.
The air was thick with cigarette smoke, the sound of clinking glasses and Chinese pop songs.
The person who turned up was not who we expected. A skinny young man wearing half-rim glasses and a dark trench coat, Maomi looked like he could be a college student. He said he was 27.
Showing an interest in investing in his business, Ian asked how much he made.
"Our daily turnover is around 5,000-10,000 Chinese Yuan (US$700-$1,400; £565-£1,130)," Maomi said proudly, showing the transactions on his phone. "Very stable income, right?"
Ian acted impressed, and mentioned the name Uncle Qi.
Maomi admitted: "I am Uncle Qi."
But to our surprise, he revealed Uncle Qi was not just one person.
He managed a team of 15 people, including 10 in China who made videos under the same name. Maomi received 30 to 100 videos from them each month.
The videos were then sold on the three websites which Maomi confirmed he owned. They had more than 10,000 paying members, mostly Chinese men.
"The key is to be authentic. It has to be real," Maomi said. He later told us his websites even sold videos of drug-facilitated rape.
Maomi talked about his business as though it were any other budding start-up. He described his team as "passionate" and "brave". He even casually mentioned he had been training others to carry out and film sexual assaults.
But there was one thing he never mentioned - the women in his videos. It was as if they didn't matter to him at all.
We wanted to know Maomi's real identity. At another meeting with Ian, he opened up about how he got into this business.
Like many boys, Maomi liked Superman, anime and video games growing up. But when he was 14, he started watching sexual assault videos like the ones he sold now.
He knew his business was not risk-free.
"I am so cautious," Maomi said. "Safety first." To avoid scrutiny from the Chinese authorities, he planned to naturalise as a Japanese citizen.
However, as careful as Maomi was, he made a mistake.
When Ian asked where to send the investment funds, Maomi pulled out his bank card and handed it to Ian.
The card revealed his real name - Tang Zhuoran.
Later, we confronted Maomi with our allegations.
As we approached, he tried to cover his face and walked away. And all of a sudden, he snapped, hitting out at our camera and crew.
The next day, by coincidence, we spotted Maomi at the airport. He was leaving Japan.
Uncle Qi's Twitter account, where he openly promotes the abuse videos, is still active.
Twitter did not respond to our request for comment. Instead, they sent us a poo emoji, which has been an automatic reply to any inquiry directed to their press email since March.
We also put our allegations to Noctis and Lupus. They did not respond. We have since learned they no longer work with Maomi.
On a spring day, we meet up with Takako to tell her about our investigation. Appalled, she says: "We women are just content in their videos. They see us as objects. They don't think we have a heart."
Takako advocates for tougher laws against these crimes.
Japan is set to reform its sexual assault laws. However, campaigners say these changes don't go far enough.
But Takako will not give up. "We will not cry ourselves to sleep."
You can watch the full film in English on the BBC World Service YouTube channel.
Additional reporting by Chie Kobayashi, Ryuzo Tsutsui, Hanae Arrour Takahashi and Joel Gunter
If you are affected by the issues in this story you can contact the BBC Action Line.