A 20-year-old gunman shot and killed his mother at their Newtown home, then 20 first graders and six adults at the nearby Sandy Hook Elementary last Friday before he committed suicide.
Jenner, a 63-year-old longtime Los Angeles resident, graduated from Newtown High School and the school's sports stadium now bears his name.
The champion runner, whose younger sister Lisa attended kindergarten at Sandy Hook, said what happened in his former community last week is "unimaginable."
"Not in a million years would you ever think that this quiet little town would have such a terrible, terrible tragedy happen there," Jenner told E! News. "You just can't believe it ... I just get mad inside at humanity sometimes and how a twisted mind can be so evil."