Asked if he'd talked with victims, Trump added: "I did indeed, and it's very sad something like that could happen".
After the Orlando shootings at a gay nightclub that left 49 dead in June 2016, then-candidate Trump tweeted, "Appreciate the congrats for being right on radical Islamic terrorism".
In the first hours after the Parkland shooting, the only statement from the White House was that Trump was "aware" and monitoring the situation.
Trump is expected to meet with officials from the Broward County Sheriff's Office later Friday.
Postings under the name Nikolas Cruz included threatening comments under videos on YouTube and other sites, including "I whana shoot people with my AR-15" (sic), "I wanna die Fighting killing s**t ton of people" and "I am going to kill law enforcement one day they go after the good people".
And don't you dare let anyone say it's too soon to be talking about it, because you said it after Vegas, you said it after Sandy Hook, you say that after every one of these eight now-fatal school shootings we had in this country this year. "It's a dream. It hasn't hit me yet".
Other lawmakers have also called for gun control. An estimated 8,000 people gathered to remember the victims at the Thursday night vigil.
The University of Sydney, in a 2016 study published in the Journal of the American Medical Association, found Australia hadn't experienced a fatal mass shooting - one in which five or more people are killed - since the 1996 shooting. "So, here's what you do to fix that", he said.
During his speech Trump said he would be meeting with governors and attorneys general to discuss school safety, but he offered no hints about what type of action he might call for.
Rubio was in his full NRA damage-control mode by Wednesday evening, telling Fox News that we need to slow down, take our time, before we "jump to conclusions that there's some law we could have passed that would have prevented it".
At least 21 people have been killed and more than 19 injured in the shootings.
Trump ignored shouted questions from journalists as he departed about a report in The New Yorker magazine that he had affair in 2006 with a Playboy model.
"On behalf of the students and employees at Jenks Public Schools, our thoughts and honest condolences go out to the victims and the families of the tragic school shooting in Florida". There was also a measure banning bump stocks, which allows a weapon to fire more rapidly, and another bill to bring back the one-gun-a-month purchase law, which was repealed years ago.
Lemire contributed from NY.
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