Marcus Hutchins is the 23 year old security researcher behind the @MalwareTechBlog Twitter account; he's the guy who figured out that the Wannacry worm had an accidental killswitch built in and then triggered it, stopping the ransomware epidemic in its tracks. According to a US Marshals spokesman, Hutchins was arrested by the FBI shortly after the Defcon/Blackhat conference in Las Vegas, though no one has disclosed the charge. His friends cannot locate him.