UNITED STATES (ORLANDO FLORIDA)  - A heavily armed assailant opened fire in a packed Orlando nightclub early Sunday in a bloody massacre, killing 50 people and prompting a terrorism investigation, authorities said.  Mayor Buddy Dyer said 50 people were killed and dozens more wounded in the attack at Pulse Orlando, a gay club just south of downtown. The shooting began at 2:02 a.m., when three police officers engaged in a gun battle with a suspect outside the club. A hostage situation then took place inside, and a SWAT team was called in, Mina said. Police received updates from patrons trapped in the club, and decided to storm the club at about 5 a.m.