Bombers knock-off jihadists: Biden 'We refuse to bend': US Vice President Joe Biden speaks during a memorial service for fallen MIT police officer Sean Collier. US Vice-President Joe Biden has denounced the Chechen brothers accused of planting the Boston Marathon bombs as "two twisted, perverted, cowardly, knock-off jihadists," even as investigators scramble to find out more about the suspects, one living and one dead. Speaking on Wednesday at a service for slain Massachusetts Institute of Technology police officer Sean Collier, Mr Biden explicitly connected the bombings and the officer's slaying to the US war against terrorists. "They know they can never defeat us," he said. It infuriates them that we refuse to bend, refuse to change, refuse to yield to fear. Investigators believe officer Collier was shot dead by one of the brothers days after the bombings. Dzhokhar Tsarnaev, 19, has been charged with use of a weapon of mass destruction and destruction of private property with an explosive for the April 15 blasts, which killed three people and injured more than 260. Tamerlan Tsarnaev, 26, died after a shootout with police. Since the Tsarnaevs' uncle, Ruslan Tsarni, stood outside his home on April 19 and apologised to the victims on behalf of his suspect-nephews, Americans have been writing in to offer their support to him, his wife and their six children. He said the brothers were "losers not being able to settle themselves and thereby just hating everybody who did." In one of many such letters, a person from Colorado wrote: "You may get 'hate mail', so please consider this 'love mail'. Thank you for your courage in coming forward. MCT, Bloomberg