In a bizarre incident, an unidentified person killed three passengers and injured another when they were travelling in the Chennai-bound APSRTC bus at Tada in Nellore District, early today. Sullurpet CircleInspector Hanumantha Rao told mediapersons that the unknown person, suspected to be a psycho, throttled the neck of one of the passenger and inflicted knife injuries in the chest of other two passengers when they were asleep. When the injured person raised an alarm, the unidentified person alighted from the running bus and escaped under a the cover of darkness. Two of the passengers died on the spot while another succumbed while undergoing treatment at Sulurpet Government hospital where the injured person was admitted in a critical condition. The bus was proceeding to Chennai from Bhadrachalam and the incident happened at 0400 hrs.