When I taught computer science, often on the first day of class, once my excited nerdlings had sat themselves down in front of a computer to begin their quest to become the next Bill Gates and conquer the world, I would flick the classroom lights off and on several times.

LONDON (AP) — Four people alleged to be part of an organized crime ring were arrested Thursday for damaging cyber attacks that hit British retailers Marks & Spencer, Co-op and Harrods, the National Crime Agency said.

The unnamed suspects were identified as British males aged 17 and 19, a 20-year-old British woman and a 19-year-old Latvian man. They were arrested on suspicion of blackmail, money laundering, crimes for violating the Computer Misuse Act and participating in an organized crime group.

M&S said the cyberattack in April stopped it from processing online orders, left store shelves empty and cost it about 300 million pounds ($407 million).

Supermarket chain Co-op said attackers stole customers' personal data, disrupted payments and prevented it from restocking shelves. Luxury London department store Harrods restricted online access in May after it was unable to process orders.