The suspect has been identified as 19-year-old former student Nikolas Cruz, who was arrested after a brief manhunt.
Trump had tweeted early this morning, "So many signs that the Florida shooter [Nikolas Cruz] was mentally disturbed" - and he stuck to that Thursday, saying he was committed to improving mental health programs.
"Neighbours and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"
A day after 17 people lost their lives in a storm of bullets at a South Florida high school, police are still trying to piece together what happened.
The sheriff said the shooter used an AR-15 rifle and had "multiple magazines".
It was the deadliest ever at an American high school, surpassing the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers killed 12 students and a teacher and then themselves.
"We'll provide continuous support for all of the students and families impacted by this heartbreaking situation".
He added that the school had sent out an email warning teachers about Mr Cruz's threats.
It's not the first time the president has made a case for mental health treatment and awareness in the aftermath of a deadly shooting.
In the wake of America's latest mass shooting, a local journalist is watching messages appear online, from people desperate to find their loved ones. But there's no evidence so far to suggest the kind of mental illness diagnosis or involuntary commitment that would prevent someone from legally purchasing a firearm. "We are responsible", Murphy said. Trump also failed to include the fact that there was an armed officer patrolling Marjory Stoneman Douglas High School at the time of the shooting. Officials said another 14 were seriously injured.
Cruz's father died of a heart attack in 2005 when he was still a young child.
Israel says investigators are dissecting the suspect's social media posts and found material that is "very, very disturbing".
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