Delhi Police have identified a mentally disturbed youth from Uttar Pradesh as the murderer of college girl Radhika Tanwar. She was shot dead in broad day light on Tuesday. Deputy Commissioner of Police (South) H G S Dhaliwal told reporters that the killer Ram Singh nurtured deep grudge against Radhika after he was beaten up for harassing her and used to tell his friends that whenever he gets an opportunity, he will take revenge.Investigators zeroed in on Singh after investigators detained his two friends — Tabrez and Ashraf — from UP's Sitapur earlier in the day. The duo allegedly harboured and helped him flee the capital yesterday.Search is on to apprehend Ram Singh.