On April 20, 1999, two teenage boys dressed in black trench coats went on a killing rampage at Columbine High School in suburban Denver.

They shot and killed 12 classmates and a teacher and wounded two dozen others before taking their own lives.

It was a sunny Tuesday morning, only 17 school days before graduation.

Outside the school, two disaffected young men knew something their classmates didn't. Dylan Klebold and Eric Harris had endgame in mind.

At about 11.15am, the two young men opened fire in the parking lot.

Senior Wade Frank, 18, outside in the parking lot, hears popping sounds and sees a girl lying against a curb, shot in the leg.

As he watches, another youth is shot in the back and falls forward.

Then one gunman throws a bomb into the parking lot and heads inside.

"He was just casually walking. He wasn't in any hurry," says Frank.

"One boy was running and suddenly his ankle just puffed up in blood," says sophomore Don Arnold, 16.

"A girl was running and her head popped open."

As the gunmen walk into the school, two students lie dead outside. Still shooting, the two walk to the cafeteria, where food server Karen Nielsen hears someone yell, "Get down!"

Klebold, 17, and Harris, 18, are heavily armed - an assault rifle, sawed-off shotguns, handguns. In the cafeteria, one removes his trench coat to reveal home-made grenades. He tosses a pipe bomb.

Gunshots echo. Students fall. One gets up to run and others follow.

Many of the 900 students in the building duck into closets and bathrooms, under tables and chairs. A couple call 911 with mobile phones. Dozens flee the building and hide in brush around the school.

Senior Nick Foss, 18, and a friend push two teachers, a cook and another woman into a bathroom. The attackers bang on doors, yelling: "We know you're in there."

Casey Brackley, 15, is in the gym when an administrator herds kids into the equipment room.

They stay for 15 minutes before the administrator directs them outside.

Neil Gardner, the Jefferson County sheriff's deputy assigned to the school full-time, hears shots and spots one of the gunmen in a first-floor hallway. He radios for backup and returns fire as bullets ricochet off lockers.

Within minutes, seven officers arrive and begin pulling students, including a few shooting victims, from the building.

In the choir room, above the commons, Stephanie Williams and her classmates hear the sounds.

Someone comes to the door and, with a thumb-forefinger gesture, gives them a warning: gun.

Her teacher tells everyone to sit. But in moments, the school's two-level auditorium next door seems a safer place, so some go; then, after about 10 minutes, they run into the main hall.

As they flee, a door behind them explodes in gunfire.

Sarah DeBoer, 16, lies on the cafeteria floor until she hears a car explode outside. Then she runs into the auditorium and lies down between seats.

Fellow students - 15, maybe 20 - cry softly. Teachers warn them to be silent. In the distance, they hear sharp reports and dull explosions. Finally, a janitor enters and tells them: Go!

They run, and gunfire follows.

The gunmen head upstairs toward the library.

"All jocks stand up! We're going to kill every one of you," one gunman yells in the library.

Student Aaron Cohn, a ballplayer, is spared because a girl leaps on to his back while he lies on the floor, covering the baseball slogan on his shirt.

"They were laughing after they shot," Cohn says. "It was like they were having the time of their life."

Some students are slain at their desks, one with pencil still in hand.

The gunmen play "peek-a-boo" with others, finding them cowering under desks and opening fire. Isaiah Shoels, who is black and has tangled with the gunmen before, is one of those to fall.

Says one assailant: "Oh, my God. Look at this black kid's brain. Awesome, man!"

Some kids play dead. By the time it is over, 12 aren't playing.

Klebold and Harris leave behind shattered windows, bloody floors and a quiet unlike any the library has ever heard.

Elsewhere upstairs, coach William Sanders has been shot twice in the chest but manages to get students down a hallway away from danger. He stumbles into a science room, bleeding and coughing blood, where he dies.

Outside, the first SWAT team is on the scene 20 minutes after the first 911 calls, joining the sheriff's deputies. It finds several explosive devices around the school and treads cautiously.

About 45 minutes after the shooting begins, at noon, ambulances take the first wounded students to hospitals.

Bomb teams, fire trucks, more SWAT units and paramedics arrive.

Nick Foss and other students manage to crawl into a space between the ceiling and acoustic tiles. Foss falls through a tile, crashing on to the floor of the teacher's lounge. He runs.

Kammi Vest, 18, hides in the choir-room closet with up to 60 other students. Others try to crawl through heating vents to safety.

Shots are heard until almost 12.30pm. About that time, in the library, Klebold and Harris turn their guns on themselves, though no one will be sure of this for hours.

As 12:30 passes, after no shots echo for several minutes, SWAT teams begin sweeping the building room by room. It is, quite literally, a minefield: dropped backpacks are everywhere, each a potential bomb.

In the coming days, bombs will turn up in various shapes and sizes. They include two 16kg propane bombs hidden in the school's kitchen.

At about 2.30pm, SWAT teams begin freeing those in hiding.

At 4.30pm, with the gunmen's bodies found, authorities declare the school under control.

The bodies of the dead stay there for an entire day, until the known bombs are cleared.

Australian Associated Press