A third huddled with his classmates, silencing his phone to avoid the attention of the gunman who killed 17 people at Marjory Stoneman Douglas High School in Florida on Wednesday.
Trump said he'll travel to Florida meet with victims' families, explore how to better secure schools and "tackle the hard issue of mental health".
The order also includes flags at embassies, consular offices, military facilities and other federal installations overseas.
Through his presidency, Trump also removed more people from the NICS, effectively allowing them to buy guns: About 500,000 "fugitives from justice", or people who have crossed state lines to avoid prosecution, were allowed to buy guns.
"I want to speak now directly to America's children, especially those who feel lost, alone, confused, or even scared".
We must also work together create a culture in our country that embraces the dignity of life, that creates deep and meaningful human connections, and that turns classmates and colleagues into friends and neighbors. "Must always report such instances to authorities, again and again", the president tweeted Thursday.
Sen. Chris Murphy (D-Conn.), who is a vocal proponent of gun control, spoke on the Senate floor as news of the shooting broke.More news: Record number of Afghan civilian casualties in attacks in 2017
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Broward County school superintendent Robert Runcie said police had received "no warnings, no threats", about an imminent attack.
Dr. Evan Boyer, the medical director at Broward Health North's Department of Emergency Medicine, told reporters that the suspect was among 17 patients taken to local hospitals after the shooting.
Following a 2012 shooting at Sandy Hook Elementary School in CT that left 20 children and six adults dead, Obama announced that then-Vice President Joe Biden would lead an effort to develop a policy toward reducing gun violence.
There were some discussions at the White House on Wednesday night of a presidential statement on the shooting, but ultimately Trump and his team decided it was too early in the proceedings for him to appear on-camera, according to an official.
The creator of the video tipped off both the Federal Bureau of Investigation and YouTube, BuzzFeed said.
Senate Majority Leader Mitch McConnell, a Republican, did not mention guns as he said the Senate would observe a moment of silence at noon. But I won't see you tomorrow, or next week, or next month.
He had mixed in with students fleeing the school before being caught, officials said.