Police in southern Florida say the latest mass shooting at a U.S. school involved a former student firing outside of a high school, then continuing to shoot inside the building before eventually blending in with a group of students as they fled.
Authorities identified the shooter as Nikolas Cruz. As of late Wednesday, they said he had killed at least 17 people and sent more to the hospital.
WATCH: Florida Sheriff Calls Shooting 'Catastrophic'
WATCH: Florida Sheriff Discusses Shooter's Age, Weapons
Local officials are due to give a new update Thursday morning on the attack at Marjory Stoneman Douglas High School in Parkland, about 70 kilometers north of Miami.
Cruz was taken into custody without incident after what Broward County Sheriff Scott Israel called a "horrific, homicidal, detestable act."
Israel told reporters the shooter was armed with an AR-15 rifle and multiple ammunition magazines.
"I'm absolutely sick to my stomach to see children who go to school armed with backpacks and pencils lose their lives," Israel said. "This nation, we need to see something and say something. If we see different behavior, aberrant behavior, we need to report it to local authorities."
Investigators have looked into Cruz's online activity, including his social media accounts. Israel said late Wednesday that probe had turned up "very disturbing" things, without giving details.
He said the victims of the shooting included both children and adults.
Governor: shooting ‘pure evil’
Before going to a hospital to meet with victims and their families, Florida Governor Rick Scott gave his reaction to the attack, saying it was "absolutely pure evil."
"The first you think about is, you know, God I hope this never happens to my family. Then, you think about, you're furious," Scott said. "How could this ever happen in this country? How could this happen in this state? This is a state that is focused on keeping all of our children safe."
The state's Attorney General Pam Bondi said Florida will pay for the funerals and counseling for the families. The local school system is also making grief counselors available for students.
Students told television networks Wednesday unfolded like a normal day of school until a fire alarm sounded. They said they then heard gunshots, sending the students either running from the building or trying to hide.
Video taken inside a classroom showed students crouched under desks, screaming in shock and terror while gunshots sounded. Other students hid in closets, using their cellphones to text emergency messages to their parents.
One student told reporters that the suspect, Cruz, had a reputation in school of being "mentally unstable" and that he had threatened other students.
"He was definitely not the kind of person who should have been allowed to have a gun," the student said.
Trump offers federal help
President Donald Trump has offered Florida authorities all the federal help they need and tweeted his “prayers and condolences” to the victims and their families.
“No child, teacher or anyone else should ever feel unsafe in an American school,” he tweeted.
Prior mass shootings have brought calls for tighter gun controls in the United States. Trump said after a November attack at a Texas church that stricter laws would not have stopped that gunman.
Democratic Senator Chris Murphy of Connecticut was among those Wednesday who urged more concrete action, blaming Congress for what he called a "scourge" of school shootings.
“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.
He added on Twitter, "If you're a political leader doing nothing about this slaughter, you're an accomplice."
Connecticut is home to Sandy Hook Elementary School in Newtown, where 20 elementary school children and six adults were massacred in a 2012 shooting.
VOA's Michael Bowman contributed to this report.