PARKLAND, Fla., Feb 14 - A 19-year-old gunman opened fire on Wednesday at a Florida high school he had previously been expelled from, killing 17 people before he was arrested by police, authorities said.The violence erupted shortly before dismissal at Marjory Stoneman Douglas High School in Parkland, about 45 miles (72 km) north of Miami. Live television footage showed students streaming out of the building as dozens of police and emergency services personnel swarmed the area.The gunman was identified as Nikolas Cruz, who previously attended the school and was expelled for unspecified disciplinary reasons, Broward County Sheriff Scott Israel said at a news briefing hours later."It's a horrific situation," Broward Country Schools Superintendent Robert Runcie told reporters separately.The gunman surrendered to police without a struggle, Israel said."It's catastrophic," he said. "There really are no words."Twelve of the dead were killed inside the school building, two others just outside, one more on the street and two other victims died of their injuries at a hospital, Israel said. He said the victims comprised a mixture of students and adults.It was the 18th shooting in a US school so far this year, according to gun control group Everytown for Gun Safety. That tally includes suicides and incidents when no one was injured, as well as the January incident when a 15-year-old gunman killed two students at a Benton, Kentucky, high school.More than five years have passed since a gunman killed 20 first graders and six educators in Newtown, Connecticut, stirring the long US debate about gun rights, which are protected by the Second Amendment of the U.S. Constitution.