The worst mass shooting in U.S. history is raising new questions about the safety of the homeland and shaking up the race for the White House. Shocking revelations that the shooter had been previously investigated by the FBI more than have many asking whether the incident could have been prevented. At least 50 people were killed and 53 were wounded or injured in the shootings early Sunday morning. The shooter, Omar Mateen, pledged allegiance to the Islamic State in Iraq and Syria (ISIS) before attacking Pulse, a gay nightclub in Orlando, with an assault rifle and a handgun. The FBI...