Update February 14, 8:20 PM: In addition to at least 17 people killed, law enforcement officials said that at least 15 people had been sent to the hospital with injuries in Wednesday’s shooting at a Florida high school.

Further details emerged of how the shooting occurred on Wednesday evening. Florida Sen. Bill Nelson told CNN that he had been briefed by law enforcement officials who described what seemed to be a well-planned assault.

“The shooter wore a gas mask and he had smoke grenades,” Nelson said. “He went and he set off the fire alarm so that the kids would come pouring out of the classroom[s] into the hall. And there the carnage began.”

Update February 14, 6:30 PM: At least 17 people have been killed in Wednesday’s shooting at a Parkland Florida high school, according to law enforcement officials.

The suspected shooter is a 19-year-old former student at Marjory Stoneman Douglas High School named Nikolas Cruz, who is now in custody. The death toll makes this the deadliest high school shooting in U.S. history. (The AP and the New York Times initially identified the gunman’s first name as spelled “Nicolas,” while other outlets including the Washington Post identified the spelling as “Nikolas.”) It supplants the 1999 Columbine massacre, in which 13 people—including 12 students—were murdered and two perpetrators took their own lives.

“At this point we believe he had one AR-15 rifle,” Broward County Sheriff Scott Israel told reporters of the alleged shooter.

Update February 14, 4:35 PM: Several outlets have picked up videos and social media posts that have been shared by people who were in the building during the shooting. The following video contains gunshots.

VIDEO: Students scream as gunman fires several gunshots pic.twitter.com/VUYMBodU9m — News This Second (@NewsThisSecond) February 14, 2018

President Trump has tweeted condolences for the victims and their families.

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. — Donald J. Trump (@realDonaldTrump) February 14, 2018

White House Press Secretary Sarah Huckabee Sanders announced that Wednesday’s press briefing would be cancelled in light of the shooting.

Update February 14, 4:25 PM: The Broward County Sheriff’s Office has tweeted that the shooter is in custody and that they’ve counted 14 “victims” so far.

So far we have at least 14 victims. Victims have been and continue to be transported to Broward Health Medical Center and Broward Health North hospital. #StonemanShooting — Broward Sheriff (@browardsheriff) February 14, 2018

Shooter is now in custody. Scene is still active. #stonemanshooting — Broward Sheriff (@browardsheriff) February 14, 2018

Initial post: At least one person is reported dead and 20 more have reportedly been injured in a mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida. From 7 News Miami:

Margate Fire Rescue has deemed this a mass causality incident, which means at least 20 people have been injured. […]As 7 Skyforce arrived on the scene, one person was seen being wheeled into a Coral Springs Fire Rescue ambulance, but he was on his phone. However, soon after, at least four other people were spotted surrounded by fire rescuers, at the corner of an intersection at the north part of the school

Video has been posted on social media of students fleeing the scene.

Students at Stoneman Douglas High School in Parkland, Florida flee amid reports of a shooting at the school. https://t.co/O0pXTKDQ3y pic.twitter.com/s53xK8mcBp — ABC News (@ABC) February 14, 2018

A mother of the students told NBC News that her daughter called her from inside the school and told her that she was barricaded inside with around 20 other teachers and students.

JUST IN: Mother of one of the students at Marjory Stoneman Douglas High School tells NBC News' @TammyLeitnerNBC that students are still trapped inside the school. pic.twitter.com/bEjZFRoNTa — MSNBC (@MSNBC) February 14, 2018

Correction, February 14, 6:44 P.M.: This post originally misstated that the Columbine mass shooting occurred in 1998.