PARKLAND, Fla. (Reuters) - A 19-year-old gunman returned to a Florida high school where he had once been expelled for disciplinary reasons and opened fire with an assault-style rifle on Wednesday, 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 Nikolaus 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.