A college student was killed on a train in San Francisco . DA : " I have no idea why this happened, but it is not the first time that someone has been shot and killed by an unknown assailant while riding BART. " Nobody noticed the gun before the shooting , DA says .
A college student was killed on a train in San Francisco . DA : The gunmen waved his gun around and showed it before the shooting . Nobody noticed the gun before the shooting , DA says .