A student who witnessed the incident said he had to hide in a closet while the school was under rampage by the gunman.
Fifteen people were killed at the school itself and 17 were taken to hospital, two of whom died of their wounds, the sheriff's office said.
An ex-classmate and former teacher told ABC News that before Cruz was expelled from the school he was barred from carrying a backpack on campus. He opened fire as classes finished for the day. According to Jillian Davis, a 19 year-old recent graduate from the high, Cruz had odd affinity to knives and guns, adding that no one ever took him seriously.
The Sun Sentinel reported that Feis, acting as a school security guard, responded to the original call on a school walkie-talkie.More news: Jamaal Lascelles slates 'missing' Manchester United defence during Newcastle defeat
"Today I speak to a nation in grief", the president said. It is not known yet if the shooter was a student at the school.
The White House cancelled its daily press briefing following the news and President Donald Trump has spoken with Florida governor Rick Scott.
He earlier tweeted his condolences to the families of the victims. "RIP to victims & prayers up for the families in Fl", the father of one wrote on Wednesday night, speaking on the tragedy that unfolded at Marjory Stoneman Douglas High School just before 3 p.m. on Wednesday.More news: Lebanon tells USA committed to calm at southern border