LONDON: Four people under 21 have been arrested as part of a police investigation into cyberattacks that disrupted the operations of UK retailers Marks & Spencer, the Co-op and Harrods, Britain's National Crime Agency said on Thursday (Jul 10). April's ransomware attack on M&S, one of the best-known names in British business, was the most serious, forcing it to suspend online clothing shopping for nearly seven weeks and costing it about 300 million pounds (US$400 million) in operating profit.