It happened on April 20, 1999 in Littleton, Colorado by Dylan Klebold and Eric Harris, two senior students at Columbine. It happened in the middle of the day with guns, knives and multiple bombs killing 12 students, 1 teacher and the two murderers themselves.
Klebold and Harris met at Ken Caryl middle school in 1993 and both could not make friends in high school. They both got picked on and worked at a pizza parlor together and playing a video game called Doom during their free time. Klebold thought of suicide in 1997 and both boys started planning the massacre in 1998. They had a history of breaking into cars. Their record was expunged after 11 months of workshops, counselors and volunteer projects.
Klebold and Harris hated everyone except a select few people. On Harris's journal he wrote that he hates racists, martial arts experts and people who brag about their cars. He said that Star Wars fans needed to get a life. He stated that he hated people who mispronounces words. He hated slow drivers and the WB Network.
In yearbooks they had drawn out their plots and the people that would die in 1998. There was history found on their computers where they had researched how to make homemade bombs and other explosives. They had eventually got an arsenal of guns, knives, and 99 explosives. There was evidence that they studied the cafeteria nothing that after 11:15 there would be over 500 people. They planned to plant propane bombs to explode at 11:17 and shoot any survivors running out. There was proof that they boys had planned on it happening on April 19th which is the anniversary of the Oklahoma city bombing or April 20th being the 110th anniversary of Adolf Hitlers birthday.
They were friends with those that were in the "Trench Coat Mafia", but were not part of it. They only wore trench coats that day to conceal their weapons.
They arrived at school at 11:10am. At 11:14 they went after the cafeteria carrying 2 20-lb bombs in duffel bags times to go off at 11:17. They went back to their cars and waited for the explosion that didn't go off that would have killed 488 people. Klebold armed with a 9-mm semi-automatic handgun and a 12-gauge double-barrel sawed off shotgun and both boys had utility belts filled with ammo. Klebold had a black glove on his left hand and Harris one on his right hand. They had knives, backpacks and duffel bags full of bombs.
11:19 the 2 pipe bombs they had planted in a field nearby as a distraction went off alerting the police. They started firing their first shots at students outside the lunch room. Immediately, 17 year old Rachel Scott was killed and Richard Castillo was injured. Harris removed his trench coat while they kept firing. A group of boys thought it was a senior prank and paintball guns so they walked right into them; injuring them. They then shot at 5 people in the grass, injuring them. They almost continually kept throwing bombs while shooting. Some students pretended to be dead. A policeman had arrived and exchanged shots with Harris, none were injured.
11:25 They entered the school. They threw 2 pipe bombs in the cafeteria where 52 students and 4 staff were hiding.
11:29 They entered the library. They shouted at everyone to "Get Up", nobody stood up. They asked all of the white caps to stand up, who were the jocks, and nobody did so, so they started firing at everyone. Killed 3 guys under a table, then banged 2 times on a table and said "Peek-a-boo" then shot and killed 1 girl, the kick from the gun broke his nose. 10 were killed in the library, 12 were injured and 34 escaped uninjured.
11:44 they went into the hall then into the cafeteria. They tried to ignite the bombs and couldn't. They shot at the bombs causing a fire but the sprinklers put the fires out.
12 o'clock they went upstairs to the library. 12:02-12:05 they shot out the windows and toward the policeman and paramedics. 12:05-12:08 they shot themselves in their heads. Ending the massacre. It lasted from 10am to Noon. Over 2,000 students were at Columbine that day.
The remaining students were sent to the Elementary school and put on stage for parents to claim. Some were still at the high school and waited hours to be rescued. Klebold and Harris had fired 188 rounds, 76 bombs although only 30 exploded, 13 bombs in their cars and 8 at home didn't explode.
Looking back at the tell tale hints: There are sayings that the video games they played, the journals they kept, the guns and bombs in their rooms- All stuff easily found if the parents had looked in their rooms. Harris had also kept a very hateful website.