First Mass Murder With an AR-15?

If you have been working this issue as long as me, you remember when the semiauto rifle used in mass murders was the AK-47.  Now the dying legacy media click-bait with AR-15s.  This change, I think, may be because the news coverage tells the unstable who want to go out in a blaze of infamy what gun to use.  So when did the choice of unstable sociopaths change?