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I probably was sleeping.

Columbine: I was in elementary school so I have no idea what I was doing.
Virginia Tech: I was in middle school and I remember people saying oh that evil asian shot and killed people.
Sandy Hook:
I had returned home from college and saw king Obama crying on tv about sandy hook because these children would never get married. The next day on a commuter bus to college I heard people saying it was hoax to impose gun control.
Elliot Rodger: I was on summer vacation playing video games, I was trying to get 100% in gta san andreas.
Chris Haper Mercer: I was in college reading a book about sandy hook shooting. Cannot really remember anything else.

In the Las Vegas shooting I was lying on my bed playing some games and my mother appeared from nowhere and told me "Have you seen what is happening in Las Vegas right now?!"

In Sandy Hook I was at school, I do not live in America so I really did not hear about it until I arrived home and turned on my computer

Virginia Tech is kinda the same with Sandy Hook, I knew about the shooting a long time after it happened

Elliot Rodger's case I really do not remember what I was doing

Sleeping

Actually, I was at the funeral for my partner's father when a lot of the news (death toll, injured, etc.) started coming in. It was also my birthday that day. In a way, being interested in the information kept both of our minds off of what was really happening (the funeral). It's a long story, but it was an awkward and uncomfortable funeral to begin with.

Well, I don't remember where I was during the Las Vegas shooting (probably sleeping, found out the next morning) but I do remember where I was during a couple other ones.

STEM school shooting - I was sick at home going on my computer. I saw that the shooting was in the headlines.

Charlottesville car attack - Found out about it late that night when my sister had a birthday party.

UNCC shooting - Just like STEM, I was also on my computer and saw it on the online news headlines.

I was sleeping I think