Britain's Prince William, Duke of Cambridge (L) and Britain's Catherine, Duchess of Cambridge, (L) stands with Matthew Barzun, US Ambassador to Britain, after signing a book of condolence for the Orlando shooting victims at the US Embassy in London on June 14, 2016. 
US investigators on Tuesday worked to untangle the complicated motive of the "radicalized" gunman behind the massacre at a gay club in Orlando, as reports emerged that he frequented the popular nightspot and used gay dating apps. / AFP / POOL / Philip Toscano        (Photo credit should read PHILIP TOSCANO/AFP via Getty Images)