Law enforcement from all over the U.S. were in Boston to honor Sean Collier, the university police officer authorities say was killed by the Boston bombing suspects.

Vice President Joe Biden addressed Collier's parents.

SOUNDBITE: U.S. Vice President Joe Biden Saying:

"Mom, dad, and Joe, my heart goes out to you, I hope you find some solace in this moment of extreme grief, looking around this field and listening to what is being said and written, and the response to people of this country and the around the world about your son."

Investigators have been talking to Dzhokhar Tsarnaev to try and find out who radicalized the brothers and what their reasoning was for the attack.

It was a question Biden asked the crowd at the memorial service.

SOUNDBITE: U.S. Vice President Joe Biden Saying:

"They can never defeat us, they can never overthrow us, they can never occupy us so why? Why? Whether it's al-Qaeda central out of the fatwa or two twisted, per