Also, this account suggests that maybe younger bro really did kill older bro when he ran over him with his car:
Tamerlan Tsarnaev, now out of his car, attempted to lob a makeshift bomb at police, but the device exploded in his hand. While Tsarnaez was firing a pistol with the other hand, police tackled and tried to subdue the 200-pound amateur boxer.
Dzhokhar Tsarnaev, apparently intending to help his brother, tried to ram the officers with the Mercedes. Instead, the officers lunged out of the vehicle’s path and he ran over his brother and dragged him along the street before speeding off with police in pursuit.