The Tsarnaev Brothers

15 April 2013, two homemade bombs exploded near the finish line of the Boston Marathon, killing three people and wounding 264 others. The elder of the brothers implicated in the attack, Tamerlan Tsarnaev, died in the ensuing manhunt; Dzhokhar's trial got underway in early 2015. What we don't know is why. How did such a nightmare come to pass?