Thousands attend memorial for murdered Boston policeman Sean Collier A Boston police officer shot dead allegedly by the marathon bombing suspects has been remembered at a memorial ceremony attended by the US vice-president. Thousands of police from around the country were also there to pay their respects to 26-year-old Sean Collier at the Massachusetts Institute of Technology where he worked and was killed. If the purpose of terrorism is to instill fear, you saw none of it here in Boston. Boston, you sent a powerful message to the world," Joe Biden told the mourners. You are their worst nightmare: all the things these perverted jihadis - self-made or organised - all the things they fear. The policeman's murder provoked a huge manhunt that led to one suspect's death and another's arrest. US security agents, under scrutiny over whether enough heed was paid to Russian warnings about Tamerlan Tsarnaev, are in Dagestan investigating. His brother and alleged accomplice has reportedly said they acted alone - a claim that one Republican congressman has treated sceptically. More about: Boston, Explosion, Funerals, Marathon, Terrorism, USA