By Tehmina Kazi In what appears to be the worst mass shooting in US history, 29-year-old Omar Mateen opened fire in a gay Florida nightclub at 2am on Sunday, killing 50 people and injuring up to 53 others. Orlando sheriff Jerry Demings called the shooting a “domestic terror incident” and the FBI — who already had Afghan-American citizen Mateen on their radar — are now investigating his links to terror groups abroad. His father, Mir Seddique, claimed that Mateen’s motives were not religious, and that he had been incensed…