In passionate remarks at a memorial service for slain Massachusetts Institute of Technology police officer Sean Collier, Vice President Joe Biden called the Boston Marathon bomber suspects "knock-off jihadis" and warned those seeking to terrorize America that "we are grieving, but we are not bending."

Speaking to the Collier family, law enforcement officials, and students gathered at MIT, Biden echoed a question asked across the country in recent days: Why? "Why, whether it's al-Qaeda central out of the Fatah or two twisted, perverted, cowardly, knock-off jihadis here in Boston. Why do they do what they do?" he asked.

"They do it to instill fear," he said. "To have us, in the name of our safety and security, jettison what we value most and the world most values about us, our open society, our system of justice that guarantees freedom."

Biden cautioned that "the moment we change, the moment we look inward, the moment we get into a crouch in defensive, that's the moment that they win."

"You are their worst nightmare," Biden told the thousands of mourners gathered at Briggs Field, "All the things these perverted jihadis, self-made or organized, all the things they fear."

" We will not yield to fear," he continued.

Collier, 26, was allegedly shot and killed last Thursday by the Tsarnaev brothers, as authorities were closing in on the bombing suspects. "No child should predecease their parents. I know from experience… that sense of hollowness. It feels like you're being sucked into a void you can't control. I remember," said Biden, whose wife and young daughter were killed by a drunk driver in 1972.

"I know there is nothing anyone can say or do to bring you solace, but I know from experience… that the moment will come when that thing that triggers the memory of Sean… a song, a season or a holiday or passing a Little League Field… you'll know it's going to be OK, when the first instinct is to get a smile to your lips before you get a tear to your eye," he said.