New details released Tuesday by the Florida Department of Law Enforcement, revealed how a gunman armed with a semiautomatic AR-15 assault rifle and “countless magazines” killed 17 people at his former high school in Parkland, Fla., in February.

Sawgrass Expressway

PARKLAND

Coral Springs Drive

Marjory Stoneman

Douglas High School

Exit

Exit

Gunman fired

outside and inside

this building.

Teachers and students

fled through exits

on opposite ends

of the building.

N

PARKLAND

Coral Springs Drive

Marjory Stoneman

Douglas High School

Exit

Exit

Gunman fired

outside and inside

this building.

Teachers and

students fled

through exits

on opposite ends

of the building.

N

PARKLAND

Coral Springs Drive

Marjory Stoneman

Douglas High School

Exit

Exit

Gunman fired

outside and inside

this building.

Teachers and

students fled

through exits

on opposite ends

of the building.

N

Sawgrass Expressway

PARKLAND

Coral Springs Drive

Marjory Stoneman

Douglas High School

Exit

Exit

Gunman fired

outside and inside

this building.

Teachers and students

fled through exits

on opposite ends

of the building.

N

The New York Times | Joe Raedle/Getty Images

The heavily armed man, identified as Nikolas Cruz, 19, arrived at the school in an Uber at 2:19 p.m., shortly before dismissal time. He made his way “purposefully” toward the freshman building, said a police report released Thursday.

6

Shoots at people in the hallway

and the stairwell, killing six.

8

2:27 p.m.

Drops rifle and backpack.

Runs back downstairs

and leaves building.

5

2:24 p.m.

Goes to

third floor.

7

2:25 p.m.

Tries to set up a sniper position

from the windows, but isn’t

able to shoot through

hurricane-resistant windows.

Stairs

4

Fires into two

classrooms.

Hallway

Stairs

1

2:21 p.m.

Gunman enters

building.

Entrance

2

Shoots into four

classrooms repeatedly,

killing 11 people.

3

2:23 p.m.

Goes to

second floor.

Bathroom

Office

Entrance

6

Third

floor

8

5

7

Stairs

4

Hallway

Stairs

1

Second

floor

Entrance

2

3

First

floor

Bathroom

Office

Entrance

6

Third

floor

8

5

7

Stairs

4

Hallway

Stairs

1

Second

floor

Entrance

2

3

First

floor

Bathroom

Office

Entrance

The New York Times

1 2:21 p.m. Gunman enters building.

2 Shoots into four classrooms repeatedly, killing 11 people.

3 2:23 p.m. Goes to second floor.

4 Fires into two classrooms.

5 2:24 p.m. Goes to third floor.

6 Shoots at people in the hallway and the stairwell, killing six.

7 2:25 p.m. Tried to set up a sniper position from the windows, but couldn’t shoot through hurricane-resistant windows.

8 2:27 p.m. Drops rifle and backpack. Runs back downstairs and leaves building.

According to details described by authorities, Mr. Cruz shot people in the hallways and inside classrooms. He eventually discarded the rifle, a vest, and ammunition in a stairwell, and blended in with fleeing students to get away.

On the third floor, students were leaving their classroom when they heard gunshots, according to several students.

They ran back to their classroom, and their teacher, Scott Beigel, was shot and killed as he tried to lock the door, the students said.

As the gunman walked through the building, he shot into classrooms where students were crouched in corners and hiding in closets. After the shooting ended, students ran out of the building flanked by heavily armed police officers.

There are 30 classrooms in the building, which was originally built only for freshmen but is now used by students in other grades as well, said Melissa Falkowski, a teacher at the high school. All told, there are usually about 900 students and 30 teachers in the building on a typical school day.

APTOPIX_School_Shooting_Florida.jpg
WPLG-TV, via Associated Press

Mr. Cruz, who had been expelled from the school, concealed himself in the crowd and was among the hundreds of people fleeing the building, authorities said. After leaving the school, he walked to a Walmart and bought a drink at a Subway, according to authorities. At 3:41 p.m., he was arrested by the police as he walked down a residential street in Coral Springs, just a few miles from the school. He was taken to a nearby hospital before being released into police custody.

Correction: May 3, 2018

A graphic with an earlier version of this article misstated the location where a victim was shot. Official statements from the police after the attack said that one person had been shot on the second floor. A more detailed report issued nearly 10 weeks later showed that no one had been injured on that level.