Boston bombs: Joe Biden tribute to slain police officer

US Vice President Joe Biden has paid tribute to the police officer who was shot dead during the hunt for the Boston marathon bombing suspects.

Authorities believe 26-year-old Sean Collier was killed by Dzhokhar and Tamerlan Tsanaev as they tried to evade capture.

Speaking at the memorial service at the Massachusetts Institute of Technology, Mr Biden said his heart went out to Collier's family, ``I hope you find some solace in this time of extreme grief.''