Since I've just started this, none of this will likely have any research yet, don't mind it too much. I'm just putting titles of cases I want to research.
[WIP, I'LL PROBABLY BE WORKING ON THIS FOREVER.]
On the 20th of April 1999 (4/20/99), two students of Columbine High School executed an attack on their school, code-named 'NBK'. The attack was originally intended to be a bombing, but after the bombs failed, the duo took to shooting up the school instead. It remained the deadliest school shooting in history until 2007, when it was surpassed by Seung-Hui Cho.
11:10 - Eric Harris arrives at the school in the parking lot, having missed a philosophy test, Brooks Brown speaks to him to ask where he's been. Brooks is then instructed to leave the school, which he does. Dylan also arrives, both of their cars being beside the school cafeteria.
11:14 - Eric and Dylan both enter the school, placing two propane tank-bombs in the cafeteria, set to detonate at 11:17. 11:17, the time that Eric Harris predicted the most people would be inside the cafeteria.
11:17 - The bombs don't detonate, this means Dylan and Eric now have to enter the school and kill people themselves.
11:19 - They both stand at the top of the west exterior steps. A reports says that one of the perpetrators says "Go, go!", then they take out their guns and begin the shooting. The first gunshots, toward the west doors, kill Rachel Scott (17) and injure Richard Castaldo (17 at the time), causing him to be paralysed from the waist down.
Daniel Rohrbough (15), Sean Graves (15), and Lance Kirklin (16) all come out of a door on the side of the cafeteria. All three are hit by bullets, but Daniel Rohrbough is killed.
11:22 - Students inside the cafeteria start to notice the commotion, and move to the windows.
11:24 - Dave Sanders and two other staff members begin the evacuation of students, instructing that they leave the cafeteria through any exit but the ones towards the parking lot.
11:35 - The last victim, Corey DePooter (17), is killed in the library. Then, Eric and Dylan move to the front counter of the library.
12:00 - Approximate time. The gunmen, now beside a bookshelf, commit suicide.
The 'Columbine Effect' refers to the lasting impact that the shooting has had on people, the influence they had on younger people in seemingly similar situations. More specifically, how the Columbine Massacre inspired others to commit their own murders.
One example of someone that took influence from Columbine was Vladislav Rosylakov, the perpetrator of Kerch Polytechnic Massacre (a 'copycat crime'), which took place in Crimea on the 17th of October, 2018. The influence from Columbine is clear in the presentation of the perpetrator. He wore a white shirt with black text on it, reading 'ненависть', meaning 'hatred' in Russian; black boots; and black pants. Along with that, similarly to Eric Harris, his head was buzzed and he committed suicide in the library. All these factors combined make the influence from Eric very clear, along with the fact that online he spoke about Columbine. Over texts he said (translated into English): "It'd be cool to have a massacre", "Like Columbine for example".