In this April 16, 2013 file photo, investigators examine the scene of the second bombing outside the Forum Restaurant on Boylston Street near the finish line of the 2013 Boston Marathon, a day after two blasts killed three and injured more than 260 people. During testimony Wednesday, March 11, 2015, in the federal death penalty trial of Dzhokhar Tsarnaev in Boston, an FBI agent held up pieces of the black and white backpack he said were found near the Forum restaurant, where Tsarnaev has admitted detonating one of the bombs.

Chief says he told MIT officer killed by bombers to be safe

John DiFava testified Wednesday about his last words to Collier in the federal death penalty trial of bomber Dzhokhar Tsarnaev.