In a bizzare case that left the police puzzled, a five-year-old school girl was burnt alive by some unidentified intruders under mysterious circumstances in her home at Adarsh Nagar in North West Delhi today. At the time of the incident, which took place around 10-30am, only Nishtha, a kindergarten student of Queen Mary's School, and her elder sister Divya, a Class XI student of GTB School, were present in the first floor house. Their father, who is into shares business and has no permanent job, had gone out for some work. Their brother, who is a college drop-out, was also away with a friend.