For the latest coverage of the tragedy in Littleton, Colo., see our coverage in The Boston Globe's Boston.com. Wednesday's coverage -Rampage reconstruction -Families mourn victims -Colo. gun law debate -What went wrong? -False 'warnings' on AOL -Clinton speaks on massacre -World reactions Globe coverage -Need for prevention -TV coverage -'A suicide mission' Background -Map of Littleton -List of injured -Shooting chronology -Past school shootings -What was 'Trenchcoat Mafia'

LITTLETON, Colo. -- A kid in a white shirt heaved something up onto the high school roof, and it exploded in billowing smoke. Sophomore Don Arnold thought it was just a lunchtime prank, a firecracker maybe.

Then students started falling.

''One boy was running and suddenly his ankle just puffed up in blood,'' said the 16-year-old Arnold. ''A girl was running and her head popped open'' when a bullet slammed into her skull.

They were the first to be shot in Tuesday's bloody attack at Columbine High School outside Denver.

Arnold's girlfriend, Lindsay Hamilton, 15, was inside the school reviewing for a biology test when she heard shots and explosions.

''It's a good day to die!'' she said someone yelled at one point. ''We want everyone to die!''

She and 30 other biology students holed up in the science room with their teacher for four hours while unspeakable tragedy unfolded outside.

The attack ended only when two suspects who had stalked through hallways spraying bullets apparently killed themselves in the school library just down the hall.

By the time Hamilton was reunited with Arnold -- after a SWAT team escorted her class around puddles of blood -- 15 people were dead, including the shooters.

Many students were in the cafeteria when the assault on Columbine High began. The first lunch shift started at 11:10 a.m. and was in full swing when food server Karen Nielsen heard someone yell, ''Get down!''

Nielsen said she heard shots coming from outside and rushed to a window. She saw three victims outside.

''I was on automatic,'' she said. ''It was just, 'get to the wounded.'''

One of the boys felled outside the cafeteria door had been shot in the face, another was shot in the back. The third looked dead. Nielsen ran back inside and called 911.

Sheriff's deputies responded swiftly, but at least two hours ticked by before officers surrounded the building and moved in.

''We had initial people there right away, but we couldn't get in,'' Jefferson County Sheriff John Stone said. ''We were way outgunned.''

Students described their attackers as heavily armed -- with semiautomatic weapons and an assault rifle, some said -- and one student said one of the shooters took off his ankle-length black coat to reveal what appeared to be hand grenades.

When the attackers strode into the cafeteria, ''Everyone was trying to crawl at first, but then someone got up and ran, and everybody was running,'' said freshman Chris Donnelly, 15. ''I saw someone bleeding on the floor.''

Nearby, Nielsen was hiding with several other employees in the restroom of the teachers' lounge. They cringed when the attackers banged on other doors, shouting, ''We know you're in there,'' she said.

Terrible moments passed. The shooting receded. One person ventured out hesitantly -- and saw wounded students strewn across the cafeteria. He returned to hiding in the lounge.

When Nielsen escaped about 90 minutes after the first shots, the two wounded students she'd seen were gone; the dead body was still there. According to students, the attackers next targeted the library, shooting as they walked up the stairs and down the hall.

Many hours later, after law officers had swept the school, checking every abandoned bookbag, backpack and gym satchel for explosives, the sheriff assessed the carnage on the second floor.

''The library was one of the last rooms we entered, and it was the most gruesome,'' he said.

Surrounding the bodies of the attackers, both juniors at Columbine, were the bodies of 10 of their victims.

SWAT teams began freeing clusters of panicked students and teachers from their hiding places about 2:30 p.m. Live television feeds showed them sprinting from the school, most with hands held high overhead, to holding areas where they were frisked, questioned and offered medical care.

Many survivors were reunited with anxious loved ones at a nearby elementary school. But it took a few more hand-wringing moments. Parents watched their precious children from a distance, but could not immediately speak with them or embrace them, while authorities finished their questioning on the school stage.

Other parents suffered a night of sinking desperation, their eyes scanning lists of students accounted for, either unharmed or in hospitals -- but not finding the names they love.

With the risk of rigged explosives deemed too high to continue their investigation overnight, authorities had no choice but to shut down the crime scene -- with bodies scattered throughout the Colorado high school.