A two-year investigation by Microsoft and British law enforcement has resulted in the arrest of four people in the United Kingdom involved in carrying out fraudulent technical support calls designed to scam consumers.

Included in the sting on scammers were a 29-year-old man and a 31-year-old woman from the town of Woking, Surrey and a 37-year-old man and 31-year-old woman located in South Shields, Tyneside.

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To carry out the operation, the scammers would make unsolicited calls to potential victims, often simply working their way through a directory of names. The fraudulent actors would pose as representatives for Microsoft or an affiliated company.

In some instances, the calls came to them after users received a malicious pop-up message on their computer telling them to call Microsoft’s technical support team, accompanied with a number that would direct them to the scammers.

Once the fake tech support representative had a potential victim on the line, they would to trick them into believing their machine was infected with malware. To do so, they would direct the user to the Windows Event Viewer application logs and have them look for entries with a “Warning” or “Error” message.

Such messages are entirely common, harmless and in no way an indication of malware on a machine. Less technical users often didn’t realize the trick and would fall victim to the scam, offering payment details like a credit card number or even granting the scammers remote access to their machine.

Read: FBI Warns Of Dramatic Increase In CEO Email Scams That Cost US Businesses $2.3 Billion Since 2013

Commander Dave Clark of the City of London Police and National Coordinator for Economic Crime said, “These arrests are just the beginning of our work, making the best use of specialist skills and expertise from Microsoft, local police forces and international partners to tackle a crime that often targets the most vulnerable in our society.”

Detective Superintendent Alan Veitch of the North East Regional Special Operations Unit said law enforcement in the UK is “determined to tackle online fraud” and organized crime and is devoting investigative and technical resources to help safeguard victims.

“Realizing that you’ve fallen victim to a scam is a horrible experience for anyone. Not just the loss of money but also the feeling that you’ve been tricked and that your personal information has been stolen. Unfortunately, the names of reputable companies, like Microsoft, are often used by criminals to lull victims into a false sense of security,” Hugh Milward the Director of Corporate, External and Legal Affairs for Microsoft in the UK,  said in a statement.

“That’s why we partnered with the National Fraud Intelligence Bureau to track these people down and bring them to justice. It’s a collaboration which can cohesively combat and investigate computer service fraud.”

Milward called the arrests “just the start” of an ongoing campaign against similar scams. He also warned that Microsoft tech support never cold calls users, nor does the company use pop ups on websites prompting users to call support services.

According to Action Fraud, the UK’s service reporting fraud and online crime, tech support scams are the third most reported type of fraud, with 35,000 reports through the most recent financial year. Those incidents have resulted in $26.7 million of losses.