GARDAI believe a murdered 19-year-old was knifed in the back by local thugs and have ruled out a link to a long-running feud.Officers investigating the fatal stabbing of David Byrne at a Dublin apartment complex say they do not think the incident is part of the notorious Crumlin-Drimnagh feud, which has claimed a dozen lives over the past decade.Members of the garda underwater unit have searched a stretch of the Grand Canal near the scene of the murder and recovered a number of potential weapons, including knives.A post-mortem examination at St James's Hospital yesterday confirmed that Mr Byrne died from stab wounds in the back.Gardai said last night that they were examining a number of theories on the motive for the attack on the teen but were not following any definite line.One officer said:...