What we know about massacre

Here's what we know:

The shooter

Nineteen-year-old Nikolas Cruz, a former student at the school, has confessed to the shooting.

Cruz has been described as a troubled teenager whose behavior led him to be kicked out of the school last year for unspecified "disciplinary reasons."

The FBI admitted Friday it received a detailed warning last month about Cruz's gun ownership, erratic behavior, and disturbing social media posts, as well as the potential of him conducting a school shooting.

The information was not handled appropriately, the top US law enforcement agency said, and no action was taken.

According to local newspaper reports, Cruz had been accused of threatening fellow students and bringing knives to school.

Even before he was expelled, Cruz's pattern of disturbing behavior had attracted the attention of fellow students, teachers and neighbors.

"He was in my class at the beginning of the year and when I first met him I knew that something was off about him and he was kind of weird," Manolo Alvarez, 17, told AFP.

Broward County Mayor Beam Furr said Cruz had been treated for a time at a mental health clinic, though he had not been back for over a year.

Among the posts on Cruz's social media accounts were pictures of dead animals and his gun collection.

Cruz was given up for adoption at birth in September 1998, along with a brother. The adoptive parents were a couple in their 50s, according to the Sun Sentinel, the local newspaper.

His father died when he was young, while his mother passed away last November, leaving the boys in the care of a family friend, according to the Sentinel.

A mugshot of Cruz shows a young man with chestnut hair, hazel eyes and freckles.

The shooting

Cruz arrived at the high school in an Uber at 2:19 pm, shortly before classes were scheduled to end for the day, according to the authorities.

He was dressed in a maroon T-shirt and was carrying a semi-automatic AR-15 rifle in a black soft case. He had "multiple" magazines of bullets in a backpack.

Less than three minutes after arriving at the school Cruz began roaming the hallways firing into multiple classrooms.

Terrified students barricaded the doors and hid in closets and under desks texting for help while others fled the school.

Cruz reportedly fired a total of more than 100 rounds.

At some point, a fire alarm was pulled, sending students streaming out of the buildings on the sprawling high school campus.

It is not clear whether Cruz pulled the alarm himself or if it was done by someone else.

Cruz told police that he discarded his AR-15 rifle and fled the school by blending in with other students.

After the shooting, Cruz stopped at a Wal-Mart, where he bought a drink at the Subway restaurant there.

He was in a McDonald's at 3:01 pm and was detained 40 minutes later in Coral Springs after police identified him using school security camera footage.

The weapon

Cruz carried out the shooting using a semi-automatic AR-15-style rifle that had been legally purchased, according to the authorities.

The AR-15 is a relatively lightweight civilian version of the military M-16.

The AR-15 is different than the M-16 in that the military version allows for automatic bursts of fire while the AR-15 requires single pulls of the trigger.

Magazines for the rifle generally hold 10 to 30 rounds and Broward County Sheriff Bobby Israel said Cruz had "countless magazines, multiple magazines."

AR stands for ArmaLite Rifle, the company that originally made the weapon, which costs as little as $500.

The AR-15 and other guns defined as assault rifles were banned by Congress in 1994, but the prohibition expired in 2004.

AR-15-style rifles have been used in a number of recent mass shootings including the massacre of concert-goers in Las Vegas last year, the elementary school shooting in Newtown, Connecticut, and the mass murders at a church in Sutherland Springs, Texas.

The victims

The victims of the shooting ranged in age from 14 to 49. Fourteen students, one teacher and two sports coaches were among the dead.

Chris Hixon, 49, wrestling coach and athletic director at the school

Aaron Feis, 37, assistant football coach and security guard

Scott Beigel, 35, geography teacher and cross-country coach

Alyssa Alhadeff, 14

Martin Duque Anguiano, 14

Jamie Guttenberg, 14

Cara Loughran, 14

Gina Montalto, 14

Alaina Petty, 14

Alex Schachter, 14