Massachusetts Institute of Technology Police Officer Sean Collier, 26, of...

Massachusetts Institute of Technology Police Officer Sean Collier, 26, of Somerville, Mass., was fatally shot in an encounter with the Boston Marathon bombings suspects. Prosecutors ultimately admitted they did not know which brother pulled the trigger -- Dzhokhar Tsarnaev or his older brother, Tamerlan Tsarnaev -- according to the AP. Credit: Middlesex District Attorney's Office

CAMBRIDGE, Mass. -- Vice President Joe Biden called the accused Boston Marathon bombers "twisted, perverted, cowardly," during Wednesday's memorial service for slain MIT police Officer Sean Collier, who was remembered as a dedicated public servant and gregarious campus presence.

Speaking at the outdoor service attended by 15,000 people, including police officers from Nassau County, Biden vowed the United States would not be defeated by terrorists and praised the city of Boston for being an inspiration after the April 15 explosions that killed three and injured at least 260.

"We will not yield to them," said Biden, who was joined at the campus service by Massachusetts Gov. Deval Patrick and Sen. Elizabeth Warren (D-Mass.). "If the purpose of terror is to instill fear, you saw none of it here in Boston. Boston, you sent a powerful message to the world."

Collier, 27, was fatally shot April 18 during a late-night encounter with accused bombers Tamerlan Tsarnaev, 26, who was later killed by police, and Dzhokhar Tsarnaev, 19, who faces federal charges in connection with the bombings. Local authorities have not yet filed charges in Collier's death.

Ten Nassau police officers, many using their own time off, attended the funeral.

The trip was organized by Officer James McDermott, who manages the funeral committee for Nassau Police Benevolent Association. He wore a half-dozen colored bracelets, each memorializing a Nassau officer killed in the line of duty, including Officer Geoffrey Breitkopf, who died March 2011 in a friendly fire shooting in Massapequa Park, and Officer Arthur Lopez, 29, who was fatally shot during an October traffic stop on the Queens-Nassau border.

Wednesday's service "was especially sad, especially because of his age," McDermott said of Collier.

Security included bag searches and metal detectors. Several roads surrounding the Massachusetts Institute of Technology campus, including the main bridge connecting Boston and Cambridge, were closed.

Volunteers distributed "Collier Strong" pins, a nod to the "Boston Strong" slogan that has become a symbol of the city's resilience after the bombings.

Biden also struck a softer tone, speaking directly to the Collier family and saying he understood the "sense of hollowness you feel like you can't control," hinting at the death of his first wife and daughter in a 1972 car crash. "I know from experience the moment will come. . . . You'll know it's going to be OK. You get a smile to your lips instead of a tear."

The two-hour memorial, which came a day after Collier's funeral in Stoneham, began with an honor guard of MIT officers carrying his coffin onto Briggs Field while bagpipers played. The MIT chorus sang the national anthem and James Taylor performed "Shower the People" and "The Water is Wide" with the school's symphony orchestra.

"People ask me if Sean were here today, what would he think? He would love this," said Rob Rogers, Collier's brother. "You've got sirens, flashing lights, formations, people saluting, bagpipes, 'Taps,' the American flag. . . . He was born to be a police officer and he lived his dreams."

MIT President L. Rafael Reif said Collier made a big impact on campus in his 15 months on the job with his "deep kindness, humor, enthusiasm and charm."

The officer immersed himself in the campus community, Reif said, going on hiking trips with students and joining them on runs up the 21 flights of stairs at the campus' tallest building.

"He truly was one of us," Reif said.

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