At least 17 people, including students, were killed after a former student opened fire at a Florida high school, law enforcement officials said on Thursday.
The shooting has left between 20 and 50 people hurt, local media reported, citing firefighters.
The shooting suspect, identified as Nicolas Cruz, has been taken into custody. The FBI is assisting local officials in the investigation.
"Nicolas Cruz was the killer. He is in custody. We already began to dissect his websites and social media that he was on ... some of the things that come to mind are very, very disturbing," Broward County Sheriff Scott Israel told reporters at a news conference in Florida.
Cruz, 19, was a former student of Marjory Stoneman Douglas High School in Parkland, Florida, where the shooting took place.
Israel said Cruz got expelled from the school for disciplinary reasons.
The suspect had countless magazines, he added.
"At this point, we believe he had one AR-15 rifle. I don't know if he had a second one," the sheriff said in response to a question. The suspect was treated and released to the police.
Of the 17 dead, 12 were inside the building, two were shot dead just outside the building, one was on a street outside the school, and two people lost their lives at a hospital. Israel, however, could not confirm the number of victim students.
The school has quite a number of Indian-American students and at least one student from the community was injured in the incident.
The student, a ninth grader, sustained minor injuries as he was hit by splinters. He is being treated at a hospital.
"This is a sad day for the country and the community. We all Indian-Americans are praying for the victims," Shekar Reddy, whose friend's son was among those injured in the mass shooting, told PTI.
Authorities said several victims were being treated in the hospital. Three of them are in critical conditions and another three in stable condition.
"My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school," President Donald Trump said in a tweet.
He spoke with the Florida Governor and other top officials and was seeking frequent updates on the tragic incident.
Florida Senator Chris Murphy termed it as "a horrific scene" playing out at a high school in South Florida.
"Turn on your television right now, you're going to see scenes of children running for their lives. It's what looks to be the th school shooting in this country, and we have not even hit March," he said on the Senate floor today.
The mass shooting, which is said to be 18th school shooting of the year, seems to have rekindled a debate on the controversial gun control legislation in the US.
Murphy rued at the incident, claiming it was a fallout of "our inaction".
"This happens nowhere else other than the United States of America...It only happens here not because of coincidence, not because of bad luck, but as a consequence of our inaction. We are responsible for a level of mass atrocity that happens in this country with zero parallel anywhere else," Murphy said.
Every day in this country, 46 children are shot, said Congressman Donald M Payne.
"Every day in this country, seven children die from gun violence. Those statistics do not occur in any other high-income country on this planet," he said, urging the members of the US Congress to use their "power to make the world safer for all children."
"The motives behind the tragic shooting today remain a mystery, but one thing is certain -- the loss of innocent life at the hands of a merciless gunman is heinous and despicable," said Congressman Peter Roskam.
Democratic Leader Nancy Pelosi said another American community is reeling from the horror of gun violence, perpetrated against innocent school children.
"Too many families have lost someone to the senseless epidemic of gun violence. Congress has a moral responsibility to take common-sense action to prevent the daily tragedy of gun violence in communities across America. Enough is enough," Pelosi said.