dr. robert stern, neurology professor at the boston university and the co-founder of the study of traumatic enself-lopty. we have heard about this condition, cte, how it affects athletes who have had repeated blows to the brain and how it can be fairly lethal. we haven't heard much how cte can cause someone to be potentially violent. what do you know about fla? >> we flee some of the early symptoms and be changing in behavior, impulsivity, explosivety, rage, changes in mood. we don't think that it can cause a huge change in the way someone views the world that they will then start this long process of a well-planned kind of egregious act like tamerlan did. >> if it was someone who had a proclivity already to certain beliefs, could cte have been a tipping point? everybody wants to know the why. is this something that could lead investigate to why a motive was created from somebody who might have been on the verge of becoming very radicalized? >> he's been in boxing for a long time, had been boxing. so this disease starts early. progresses as someone gets older. we don't know when symptoms be