Police are continuing to question three men after an officer was stabbed to death and four others injured during a counter terrorism operation in Manchester. Officers were trying to arrest a man in a raid linked to the discovery of the deadly poison ricin in London last week when they were attacked. Father-of-three Stephen Oake, a 40-year-old plain-clothed special branch officer, was stabbed in the chest and died soon afterwards in hospital. Three suspects, believed to be of North African origin, were arrested at the upper floor flat in Crumpsall, in the north of the city, just before 1800 GMT on Tuesday. One is being questioned on suspicion of murder. Prime Minister Tony Blair and Home Secretary David Blunkett have condemned the killing. Of the four other officers injured, three were stabbed and one suffered a broken ankle, but their injuries are not believed to be life threatening.