Ex-babysitter says Newtown, Conn. school shooter Adam Lanza's mother warned: Don't turn your back

Ryan Kraft now lives in Hermosa Beach, in Southern California.

But, he tells CBS station KCBS in Los Angeles, he was once a student at Sandy Hook Elementary School in Newtown, Conn., and babysat for Lanza when Lanza was about 9 or 10 and Kraft was 14 or 15.

Police say Lanza, 20, went on a rampage in the school Friday, killing 20 six- and seven-year-olds and six adults before taking his own life. Lanza also shot his mother, Nancy Lanza, to death in the nearby home they shared before heading to the school, authorities say.

Kraft tells KCBS when he first heard about the shooting and that Lanza was involved, "I just couldn't think for a little while. I was shaking."

He says he recalls Nancy Lanza cautioning him never to turn his back on Adam -- "to keep an eye on him at all times ... to never turn my back, or even to go to the bathroom or anything like that."

Kraft says he remembers Lanza as quiet, very intelligent and introverted, noting, "Whenever we were doing something, whether it was building Legos, or playing video games, he was really focused on it. It was like he was in his own world."

Kraft is still having trouble believing the kid he babysat could have been involved in such unspeakable horror. "I'm just numb to it, I haven't really processed the fact that this happened right where I used to be, and that, 15 years ago, it could have been me."

Nancy Lanza, says Kraft, was very involved in her children's lives and loved them very much.

He says that, rather than feeling helpless, he decided to start a fundraiser to help the children of Newton, especially the ones who will be dealing with post traumatic stress disorder.

Kraft, who moved to California after college, also wants the money to go to helping families pay funeral expenses and to help establish a scholarship fund for survivors.

Kraft's fundraising page took in more than $53,000 in one day.