Here is a rundown of how events unfolded.

All times MDT:

11 a.m. -- About 900 Columbine High School students head into the cafeteria for lunch, hit the books in the library or in classrooms.

11:30 a.m. -- Two young men wearing long black trench coats approach the school. They pull out weapons and open fire in the parking lot.

They proceed inside, to the school cafeteria, shooting as they walk, then upstairs to the library, still shooting.

As bullets ricochet throughout the building, scores of students hide in closets, bathrooms, under tables and chairs. Some students, barricaded behind a heavy door, whisper on their terror over cell-phone calls to relatives.

Dozens more flee the building, hiding in bushes or sprinting to a nearby park.

Noon -- Police SWAT teams find several explosive devices around the school. Ambulances take first wounded students to area hospitals, all of whom managed to run outside.

Anguished parents, awaiting word on their children, gather at nearby Leawood elementary school.

12:30 p.m. -- SWAT teams begin room-by-room sweep of the high school, checking every knapsack and desk.

2:30 p.m. -- SWAT teams begin freeing students in hiding. In small groups, the students and teachers run from the building to a holding area, where they are frisked, questioned, offered medical care and then bused to Leawood elementary to be reunited with parents.

4 p.m. -- Sheriff's spokesman says as many as 25 people may be dead, a combination of students and teachers. (The final count, announced Wednesday, is 15.) Officers searching the library find the bodies of both suspects, who apparently died of self-inflicted gunshot wounds.

4:30 p.m. -- SWAT teams declare school safe.

6:15 p.m. -- Authorities find an explosive device in a car in the school parking lot. Sheriff's spokesman says entire school is regarded as a massive crime scene; campus marked off with yards of yellow tape. All bodies still inside.