New Delhi, May 4 -- A blaze tore through a residential building in east Delhi's Vivek Vihar early on Sunday, killing nine people from three families and critically injuring another, the latest in a series of major infernos in the Capital this year that have raised fresh questions over fire safety in residential buildings. The fire is suspected to have started after a short-circuit in the outdoor unit of an air-conditioner on the second floor of the four-storey building in B block of Vivek Vihar. Delhi Fire Services officials said the short-circuit likely triggered an explosion, helping the flames spread rapidly and engulf the upper floors. Rescue personnel from DFS and the Delhi Police said several residents were trapped in by the building's security set-up - a central-locking system that possibly kept the doors locked, a locked metal gate that cut off the terrace and iron grilles installed across the rear facade that prevented escape through balconies. Neighbours who dialled emergency services said some initial calls were redirected to the Uttar Pradesh Police Control Room, because Vivek Vihar lies close to the Delhi border. Prime Minister Narendra Modi condoled the deaths and announced Rs.2 lakh assistance for the families of those killed. The fire department said it received the first call at 3.47am and that tenders reached the spot around 4.10am. Neighbours and eyewitnesses said that the fire broke out around 3.30am. The fire was brought under control by 6.25am and was fully extinguished by 8am, said DFS officials. By then, nine bodies had been recovered from the building: Shikha Jain (45), a resident of the second floor; Arvind Jain (60) and Anita Jain (58), their son Nishank Jain (35), his wife Anchal Jain (33) and their one-year-old son, all of whom lived on the third floor; and Nitin Jain (50) and Shally Jain (48) and their son Samyak Jain (25), who lived on the fourth floor. The families are not related. Shikha Jain's husband, Naveen, is critical with over 40% burns. Their two daughters survived by jumping onto mattresses laid out by locals. The building is spread over an 800 square yard plot, said officials aware of the matter. Each floor has two flats - one at the back and one at the front. Investigators said the fire originated in the rear flat on the second floor and gutted the vertically aligned units above it. Flats on the first floor and front-facing apartments were largely unaffected. The building was constructed around eight years ago, said residents. Officials aware of the matter said they were still to ascertain if all the building followed all by-laws. They ordered separate probes at the zonal and headquarters levels to check the width of the staircase, examine the installation of the grilles across four floors, and the number of units. Officials said the rear metal grille and the single narrow staircase severely hampered evacuation....