Why are mass murder and serial and spree killings called "True Crime"?

Please explain to me why are crimes that include multiple homicides and possible perpetrator suicide called "True Crime"?
I don't understund how a mass shooting or a serial killers crimes are more "True" than the rest of crimes that take place all the time?

I too have wondered this, I think it's just a phrase people use to describe what they are interested in.

@Tommy QTR
Thanks for the reply!
I think it is also a name used more by the bit crazier fangirls from some other sites than people here.

I don't think it's just murder. There are a lot of kidnapping stories that are often called "true crime" for example.

Though murder is probably the crime that receives the most attention I guess, so maybe that's a reason why it connects the most to the "true crime" label.

This sample from Wikipedia reads:

"True crime is a non-fiction literary and film genre in which the author examines an actual crime and details the actions of real people."

So, technically, it's a genre. When people say they are interested in True Crime they mean the genre. I think because of the internet (tumblr) it has spread over to a slang word, for lack of a better way to describe it. True Crime now just describes real crime in general it seems. We really aren't discussing the genre on the forum, but instead the crimes themselves. Thus, it's a true crime forum because the crime actually happened.

Because they are crimes that truly happened, unless you are Alex Jones and co? The term has been around for many years, and encompasses everything from DB Cooper to missing persons to Mandelay Bay. Is the question about a narrower use of the term?

Probably because of books, because crime books are usually fiction books (mainly those stupid murder mystery ones) so it's probably a way to distinguish them from the fictional books.