Harris and Klebold both began keeping journals soon after their arrests. The pair documented their arsenal with video tapes they kept secret.Their journals documented their plan for a major bombing to rival that of the Oklahoma City bombing. Their entries contained blurbs about ways to escape to Mexico, hijacking an aircraft at Denver International Airport and crashing into a building in New York City, as well as details about the planned attack. The pair hoped that, after setting off home-made explosives in the cafeteria at the busiest time of day, killing hundreds of students, they would shoot survivors fleeing from the school. Then, as police vehicles, ambulances, fire trucks, and reporters came to the school, bombs set in the boys' cars would detonate, killing these emergency and other personnel. In the event, the explosives in their cars did not detonate.
The pair kept videos that documented the explosives, ammunition, and weapons they had obtained illegally. They revealed the ways they hid their arsenals in their homes, as well as how they deceived their parents about their activities. The pair shot videos of doing target practice in nearby foothills, as well as areas of the high school they planned to attack. On April 20, approximately thirty minutes before the attack, they made a final video saying goodbye and apologizing to their friends and families.
In the months prior to the attacks, Harris and Klebold acquired two 9 mm firearms and two 12-gauge shotguns. Their friend Robyn Anderson bought a rifle and the two shotguns at the Tanner Gun Show in December 1998. Through Philip Duran, another friend, Harris and Klebold later bought a handgun from Mark Manes for $500.
Using instructions acquired upon the Internet, Harris and Klebold constructed a total of 99 improvised explosive devices of various designs and sizes. They sawed the barrels and butts off their shotguns to make them easier to conceal. They committed numerous felony violations of state and federal law, including the National Firearms Act and the Gun Control Act of 1968, before they began the massacre.
On April 20, Harris was equipped with a 12-gauge Savage-Springfield 67H pump-action shotgun, (which he discharged a total of 25 times) and a Hi-Point 995 Carbine 9 mm carbine with thirteen 10-round magazines, which he fired a total of 96 times.
Klebold was equipped with a 9 mm Intratec TEC-9 semi-automatic handgun with one 52-, one 32-, and one 28-round magazine and a 12-gauge Stevens 311D double-barreled sawed-off shotgun. Klebold primarily fired the TEC-9 handgun, for a total of 55 times.
On the morning of Tuesday, April 20, 1999, Harris and Klebold placed a small fire bomb in a field about three miles south of Columbine High School, and two miles south of the fire station. Set to explode at 11:14 a.m., the bomb was a diversion to draw firefighters and emergency personnel away from the school. (It partially detonated and caused a small fire, which was quickly extinguished by the fire department.)
At 11:10 a.m. Harris and Klebold arrived separately at Columbine High School. Harris parked his vehicle in the Junior student parking lot, by the south entrance, and Klebold parked in the adjoining Senior student parking lot, by the west entrance. The school cafeteria, their bomb target, with its long outside window-wall and ground-level doors, was between their parking spots.
After parking their cars, the duo met near Harris's car and armed two 20 pound (9 kg) propane bombs before entering the cafeteria a few minutes prior to the beginning of the A lunch shift. The youths placed the duffel bags containing the bombs—set to explode at approximately 11:17 a.m.—inside the cafeteria before returning to their separate vehicles to await the explosion, and to shoot survivors fleeing the building. Had the bombs exploded with full power, they would have killed or severely wounded all 488 students in the cafeteria and possibly collapsed the ceiling, dropping part of the library into the cafeteria
When the cafeteria bombs failed to explode, Harris and Klebold convened and walked toward the school. Both armed, they climbed to the top of the outdoor West Entrance steps, placing them on a level with the athletic fields west of the building and the library inside the West Entrance, directly above the cafeteria. From this vantage point, the cafeteria's west entrance was located at the bottom of the staircase, next to the Senior parking lot.
At 11:19 a.m., a witness heard Eric Harris yell "Go! Go!" The two gunmen pulled their guns from beneath their trenchcoats and began shooting at two 17-year-old students who had been sitting in the grass next to the West Entrance of the school. Rachel Scott was hit four times and killed instantly. Richard Castaldo was shot eight times in the chest, arm and abdomen and paralyzed below the chest. It is unknown who fired first or which gunman shot and killed Scott.[
After leaving the library, Harris and Klebold entered the science area, where they threw a small fire bomb into an empty storage closet. It caused a fire which was put out by a teacher hidden in an adjacent room. The duo proceeded toward the south hallway, where they shot into an empty science room. At approximately 11:44 a.m., Harris and Klebold were captured on the school security cameras as they re-entered the cafeteria. The recording shows Harris kneeling on the landing and firing a single shot toward one of the propane bombs he and Klebold had earlier left in the cafeteria, in an unsuccessful attempt to detonate it. He took a sip from one of the drinks left behind as Klebold approached the propane bomb and examined it. Klebold lit a Molotov cocktail and threw it at the propane bomb. As the two left the cafeteria, the Molotov cocktail exploded, partially detonating one of the propane bombs at 11:46 a.m. Two minutes later, approximately one gallon of fuel ignited in the same vicinity, causing a fire that was extinguished by the fire sprinklers.