Boston's Return to Normalcy Continues as Officer Is Honored Police officers arrived on Wednesday for a memorial service in honor of Sean Collier. CAMBRIDGE, Mass. - Police officers from across the country and other mourners turned out by the thousands Wednesday afternoon for a somber public memorial service at the Massachusetts Institute of Technology to honor a campus officer who the authorities say was killed by two brothers who are also suspected of detonating two bombs near the finish line of the Boston Marathon last week. Dozens of metal detectors were set up at the entrance to Briggs Field on the M.I.T. campus and the authorities shut down sections of Cambridge, including the Massachusetts Avenue Bridge, which connects Cambridge to Boston, and part of Memorial Drive, a major riverside thoroughfare. The authorities say that the campus officer, Sean Collier, 27, was shot and killed during an encounter with Tamerlan and Dzhokhar Tsarnaev, the brothers who are also accused of setting off the pair of bombs that killed three people and injured more than 260 others at the marathon. L. Rafael Reif, president of M.I.T., said at the memorial service that Officer Collier had gotten along so well with the school's student body that he had gone on hiking trips and on swing dance outings with campus groups. "In just 15 months, he had built a life with us," Dr. Reif said. He touched people across our community with his deep kindness. Vice President Joseph R. Biden Jr., whose first wife and young daughter died in a car accident in 1972, also spoke at the ceremony, telling Officer Collier's parents that he knew from experience how the family was suffering. The Tsarnaev brothers, he said, were "two twisted, perverted, cowardly knockoff jihadis." Mr. Biden praised the nation's ability to recover from such terror attacks, saying, "We have suffered. We are grieving," before adding, "But we are not bending." Hours earlier, in Boston on Wednesday, Boylston Street, the busy hub that forms the northern boundary of Copley Square, was reopened to the public as the city continued to seek a semblance of the ordinary, nine days after the bombings. Along with the return of traffic to Boylston Street, the Copley Square Central Library was reopened, and Massachusetts Bay Transit Authority trains resumed service to the Copley Square station - a voice over the loudspeaker making a familiar, if no longer mundane, announcement: "Doors open on the right." "It's like, you're back to normal," said Mike Westberry, a 27-year-old university administrative assistant. Down here, everything looks the same. It's more difficult, I think, above ground," he said, referring to the six blocks around Copley Square, where there is now a makeshift memorial of flowers, notes and other tokens that have been left by well-wishers. For Susan Christiansen, 69, Wednesday meant a morning walk down Boylston Street with her dog and three of her neighbors. They made sure a CVS drugstore was open, and then tried to recall from the surveillance video images they had seen in the media how the bombers had moved through the area before and after the April 15 attack. "Gosh, those people with the bombs were walking the same way we were walking," Ms. Christiansen said, before turning to one if her companions, Sonia Hauser, 50. Don't you feel, kind of, just wary? According to a former law enforcement source who had been briefed on the bombing investigation, the authorities recovered only one gun - a handgun - used by the Tsarnaev brothers, despite the pair engaging in what officials have described as a fierce shootout with the police as the brothers sought to escape capture on Friday. The gun's serial number was obliterated and so has not yet been traced, said the source, who spoke on the condition of anonymity. The source said that no gun had been recovered from the boat where Dzhokhar Tsarnaev had been captured while trying to hide after he had been wounded by the police. Jess Bidgood reported from Cambridge, Mass., and Erica Goode and Timothy Williams from New York. Reporting was contributed by William K. Rashbaum and Michael Cooper from New York; Michael S. Schmidt and Eric Schmitt from Washington; Serge F. Kovaleski, Michael Schwirtz, Wendy Ruderman, John Eligon and Dina Kraft from Boston; Andrew Roth from Makhachkala, Russia; and David M. Herszenhorn in Moscow. Correction: April 24, 2013 An earlier version of this article, using information supplied by the Massachusetts Institute of Technology, misstated the age of the slain campus officer, Sean Collier. He was 27, not 26.