Hello, this is William Morgan with About.com, and today I am going to tell you about the Columbine Massacre.On April 20, 1999 at Columbine High School in Littleton, CO, two high school seniors, Eric Harris and Dylan Klebold, executed a plan of mass murder, killing 12 students, a teacher, and injuring 21 others before committing suicide. The massacre at Columbine is the deadliest high school shooting in US history.Harris and Klebold arrived at the school late in the morning, armed with guns, knives and an assortment of homemade bombs. They placed two propane bombs inside the school cafeteria shortly before the lunch period began. They took up positions in the parking lots and waited for the bombs to go off, planning to shoot the students as they fled the building. When the bombs failed to explode, the two began advancing toward the building firing on the students, killing two and wounding six as they made their way to the west entrance.A teacher who was inside the building saw the commotion outside and thought it was a school video. She walked to the west entrance to tell them to stop. A student was exiting the building ahead of her when the attackers opened fire on them, wounding them both. The attackers were distracted by the police sirens of an approaching deputy sheriff, allowing them to escape into the library, where the teacher told the students to hide under their desks and then called 911. Harris opened fire on the deputy, who then radioed for help.The attackers then entered the school and began roaming the halls, firing at everyone they saw but only injuring one. As the attackers made their way to the library, they came face-to-face with Coach Dave Sanders. Sanders had evacuated the cafeteria and may have planned to do the same in the library. Sanders was shot and later died.Harris and Klebold then entered the library where 52 students, two librarians and two teachers were hiding. The two terrorized those inside, randomly shooting victims, at one point pausing to fire at police outside. After killing 10 and wounding 12, the two left the library and began wandering the school. When the two reentered the library, it had been evacuated. They fired at the police once more before killing themselves, less than an hour from the time the first shots were fired.Thanks for watching. To learn more, visit us on the web at About.com.