In this image from video, students from the Marjory Stoneman Douglas High School in Parkland, Fla., evacuate the school after the shootings Wednesday.

PARKLAND, Fla. — The young man got out of an Uber, wearing school colors. He wore a maroon polo shirt with the logo of the Marjory Stoneman Douglas High School Eagles on the sleeve.

He was carrying a black backpack and a long black bag. He was walking – “purposefully,” a police report would later say – toward the school’s “1200 building.” Three stories of classrooms, with interior hallways and few means of escape.

If the gunman, whom police identified as Nikolas Cruz, 19, was trying to disguise himself as a student, it didn’t work. He was too well known at Douglas, where staffers had been warned that the troubled former student could someday pose a risk. A staffer recognized him instantly and radioed to warn a co-worker.

But the shooter still got to the 1200 building.

A few seconds later, the same worker heard gunshots.

“Code Red!” he radioed – an all-out emergency call, meant to trigger a campus lockdown.

In the next five minutes, 17 people were fatally shot.

“I heard a girl screaming for help,” said Nathanael Clark, a student at the South Florida school. “And we can’t open the door because if we open the door then … the shooter will come inside and kill all of us. And then I heard gunshots after the screams.”

If the students felt safe before Wednesday, it was because they were prepared. There was an armed police officer on campus. The students had practiced to deal with an active shooter.

“They knew what to do. We knew what to do,” said Melissa Falkowski, a teacher at the school, talking about students and faculty members on CNN. “And, even still – even with that – we have 17 casualties.” She started to cry.

On Wednesday morning, the suspect was staying with a friend’s family, which had taken him in after Cruz’s mother died last year. Most mornings, the friend’s father drove Cruz to an adult-education class, where he was studying for a GED.

Not Wednesday.

“He said something to the effect of, ‘Oh, it’s Valentine’s Day, I don’t go to school on Valentine’s Day,’ ” said Jim Lewis, an attorney who represents the family.

That afternoon, Cruz hailed an Uber, using the online car-service app. In the car on the way to Douglas, he was texting with the friend whose family had taken him in. That friend, a junior at Douglas, was in class as Cruz neared the school. Cruz’s texts to him gave no reason for alarm.

“Hey yo, hey whatcha doin?” was the last text Cruz sent, Lewis recounted.

That was 2:18 p.m.

At 2:19 p.m., the gunman arrived, according to a police timeline released Thursday.

He walked into the 1200 building, also called the “freshman building,” because many of the classes are for first-years. Authorities say he removed an AR-15 assault-style rifle from the long black bag.

At 2:21 p.m., the shooting began. Bullets streamed from the hallway, through doorways, into classrooms on the first floor.

In a psychology class, a bullet came in through the window in the classroom door and skimmed past Meghan Hill’s ear, trailing an intense ringing sound. It struck her friend in the knee. Three others also were hit. The gunman moved on.

“No one was crying,” said Mackenzie Hill, Meghan’s twin sister, relaying her sister’s memories. The wounded were moaning.

After firing into four classrooms on the first floor, police say, the gunman went upstairs.

Mackenzie, who’d just left the psychology class to use an upstairs bathroom, saw a man with a gun at the other end of the second-floor hallway. Mackenzie said she recognized him.

“I immediately knew it was him,” she said. She remembered the boy from her middle school and from the Dollar Store in town. She recalled his terrifying Instagram posts about wanting to kill people. “I always had a bad feeling about him,” she said.

Mackenzie, caught in the hallway, rushed to the nearest classroom. She knocked on the door.

But the school had gone on lockdown. She couldn’t get in.

She knocked on another door. A teacher she didn’t know looked out, saw her and opened the door. He must have “seen the fear on my face,” Mackenzie said.

She hid in the back of the classroom, beside a desk, with students she barely knew. One girl was having trouble breathing. Mackenzie texted a photo to her parents, showing her hiding under a desk, tears falling down her face.

“I love you guys so much,” she texted, fearing it could be her last communication.

Elsewhere, entire classes were caught in the hallways: They had heard an alarm – possibly related to the shooting – and thought it was a fire drill. So they left their classrooms, dutifully following the protocol for the wrong kind of emergency.

Hearing the shots, geography teacher Scott Beigel tried to reverse course and rush his students back into the room he’d just left. He had locked it behind them.

Beigel unlocked the door.

Students ran in and cowered behind the teacher’s desk. They noticed Beigel wasn’t with them.

“My teacher is on the floor,” Kelsey Friend, 16, texted her mother. Beigel, 35, had been shot while rushing students to safety. He died.

“He’s my Superman,” Friend said Thursday. “Superman saves lives, and that’s exactly what Mr. Beigel did.”

Another school staffer – Aaron Feis, an assistant football coach and security guard – also was credited with sacrificing his life to save students during the rampage.

Elsewhere in the school, students huddled inside closets, under desks, searching for news coverage and texting parents and siblings and friends. “I love you,” Meghan Hill wrote in text messages to her sister, as they hunkered down one floor apart. “Please be safe.”

In one classroom, a student used the social-media app Snapchat to record his classmates hiding. “Our f—ing school is getting shot up,” he wrote as a caption. In the video, gunfire seems to be coming through the door into the classroom – about 18 shots, the gunman so close that smoke seemed to billow in the window behind the bullets.

“Oh my God! Oh my God!” the student screamed, along with others.

At 2:24 p.m., just five minutes after police say the gunman got out of the car in front of the school, the shooting stopped, according to an official police timeline. The rifle and backpack were ditched in a stairwell, and police say the shooter left the school in a crowd of fleeing students. In his Douglas High School polo shirt – authorities said Cruz had such a shirt from his time as a member of the school’s Junior Reserve Officer Training Corps – he blended in. The armed police officer on the Douglas campus never encountered him, police said.

The gunman walked to a nearby Walmart, police said, and bought a drink at the Subway outlet inside.

Then he left and went to a McDonald’s. He sat for a while, then moved on.

Back at the school, police began to clear classrooms, scooping up the wounded and leaving the dead. Students evacuated past bodies of their teachers and classmates, both inside and outside the building.

Nathanael Clark – who’d heard a girl pleading for help outside his classroom’s locked door – left that classroom at last. He saw a girl’s body outside. He wondered if it was the girl he had heard.

In the classroom where Mackenzie Hill had been hiding, police broke through the door. As the officers were guiding students out, they noticed something odd: One of the students had put on a bulletproof vest.

The student said he’d been given the vest by his father, a police officer. He’d brought it to school with him, just in case.

As ambulances began to bring the wounded to nearby hospitals, and students began to reunite with parents, the gunman left the McDonald’s and walked into a neighborhood about a mile and a half from campus. He was headed in the opposite direction of his home, into a dead-end neighborhood bounded by lakes, an expressway and the Everglades.

A police cruiser passed, and Officer Michael Leonard spotted him. Maroon shirt, black pants – the description matched. But Leonard said he hesitated, just for a moment. Could this really be the mass shooter he was looking for? This 5-foot-7, 120-pound teenager?

“He looked like a typical high school student,” said Leonard, of the Coconut Creek Police.

Leonard called out. The young man didn’t run. He lay down on the grass, and the handcuffs went on.

It was 3:41 p.m.