A few days back I posted a thread about John La Due- he had planned to shoot up and bomb a school but someone in the community happened to see him going to a storage locker and thought it looked suspicious, and his plan was thwarted.
WBUR interviewed him in light of this shooting- you can listen to the interview here, where he talks about what was going on in his mind that led him to plan the attack.
http://www.wbur.org/hereandnow/2018/02/26/john-ladue-foiled-school-attack
He plead to a felony charge, but did not go to prison.
here is a good article on the original plan that was thwarted:
https://www.cnn.com/2015/11/03/us/minnesota-foiled-school-massacre-john-ladue/index.html
To my knowledge, he is the only person who had planned and gotten close to carrying out a mass shooting and lived to tell us about it afterwards. Most of these folks kill themselves or get killed by cops. James Holmes is another who is alive, but he hasn't done any media interviews.