A young man killed 17 people using a semi-automatic assault rifle and wounded many others on Valentine's Day in the high school he attended and was expelled from in southeastern Florida.
The 19-year-old butcher surrendered without resisting police a few hours later.
The massacre, one of the worst in the United States in 25 years, took place on Wednesday, shortly before the final classes at Marjorie Stoneman Douglas High School in Parkland, a city of about 30.000, were completed.
Watch the video-shock recorded by a student, from the moment of the attack
The suspect, identified as Nicholas Cruz, a 19-year-old former high school student who had been expelled from school for misconduct, was arrested shortly afterwards in the nearby Coral Springs community, Broward County Sheriff said.
According to him, the young man used an AR-15 semi-automatic assault rifle, which is very easy for anyone in much of the US to buy.
Cruz, born in September 1998, had posted "very worrying" messages on social media, the sheriff said, noting that it was necessary to report such behavior to authorities.
US President Donald Trump has been "informed of an armed attack on a school in Florida," the White House said, adding that "we are monitoring the situation."
Wednesday's tragedy adds to a long line of massacres committed by gunmen in the United States in recent years. In Las Vegas, in October 2017, 58 people were killed by the bullets of a single gunman, while in 2016, 49 people were killed inside a gay club in Orlando. Gun attacks are becoming more common in schools in the United States: there have already been 18 this year, including one at Marjorie Stoneman Douglas High School. After the massacre about five years ago in Sandy Hook, a primary school in Connecticut where 20 children aged 6 and 7 were killed, the exercises for the reaction of students and teachers to armed attacks have multiplied.