In the S01E02 Gideon relates how he confronted a killer who stutters saying he knew the cause but he refuses to tell the cause to the rest of the team because they should find it out for themselves.
I didn't watch the whole episode and could not find the answer on internet.
After examining a series of killings near San Francisco area hiking trails in 1979, FBI psychological profiler John Douglas concluded that the Trailside Killer [David Carpenter] was a stutterer.