Young Harry Potter actor Robert Knox has been stabbed to death after attempting to save his brother from a knife attack in Kent, southeast England. The 18-year-old, who won the role of Marcus Belby in the forthcoming film Harry Potter and the Half-Blood Prince, sustained multiple stab wounds on Friday night, after becoming embroiled in an argument with two armed men who had threatened his 17-year-old brother Jamie. Four other men were stabbed following the brawl - which reportedly stemmed from the theft of a cell phone - at the Metro Bar in Sidcup, Kent. Knox was taken to Queen Elizabeth Hospital in Woolwich, southeast London, but was pronounced dead on arrival. A police inquiry into the murder has now been opened. A spokesperson for the authorities says, "At this early stage, we retain an open mind as to any motive, and inquiries continue to establish the full circumstances of the incident. We do not believe the incident is gang-related." Executives at Warner Bros. Studio, which produces the Harry Potter movies, have released a statement expressing their condolences to Knox's relatives. It reads, "We are all shocked and saddened by this news and at this time out sympathies are with his family."