Boston Bombing Timeline

This Monday, April 15, 2013 photo provided by Bob Leonard shows bombing suspects Tamerlan Tsarnaev, 26, center right in black hat, and his brother, Dzhokhar A. Tsarnaev, 19, center left in white hat, approximately 10-20 minutes before the blasts that struck the Boston Marathon. It's a vexing puzzle about the Boston Marathon bombings: The younger of the two accused brothers hardly seemed headed for a monumental act of violence. How could he team up with his older brother to do this? Nobody knows for sure, but some experts in sibling research say the powerful bonds that can develop between brothers may have played a role. (AP Photo/Bob Leonard)