Detective Constable Joe Ryan arrived at a red-brick manor house in Hampstead, north London, early on the morning of October 31 2018. Ryan, a veteran money-laundering investigator with the Metropolitan Police, had obtained a warrant to search the property as part of an investigation into the activities of a 36-year-old woman named Jian Wen. A China-born British citizen, Wen had tried to buy a multimillion-pound property using bitcoin whose origin she couldn’t prove.

Walking up to the house, Ryan saw Wen sitting in a black Mercedes-Benz E-Class in the driveway. He tapped on the car window and told her he needed to search the house. Wen refused. Several officers surrounded the car, as Wen sat obstinately for five minutes. “It would be much easier if we had your keys to open the door, otherwise we’ll have to break in,” Ryan said. “If you want, we can arrest you,” warned another officer.

When she finally opened the front door, Wen went upstairs towards one of the bedrooms. Inside, police found Wen’s housemate, Yadi Zhang, in bed. She spoke little English. Over the next few hours, police seized laptops, notebooks and a pink USB stick stored in a metal tin. In Wen’s bedroom, they found ￡69,000 in cash. When police noticed Wen trying to delete apps from her iPhone, they asked her to hand it over. The case of another iPhone contained two €500 bills and a handwritten note with a list of passwords.