Four days after Radhika Tanwar was killed, the Police have arrested the main accused from Mumbai.A team of the Delhi Police officials arrested 25-year-old Ram Singh alias Vijay in connection with the killing of the girl outside her college in Shantiniketan area of south Delhi's Dhaula Kuan.Investigators had zeroed in on Singh after they detained his two friends from Sitapur in Uttar Pradesh. The duo allegedly harboured and helped him flee the capital two days after Radhika was shot dead.