Indian police are investigating the cause of a fire that killed 90 people in a hospital in Kolkata when poisonous fumes spread from the blaze in the building's basement. Patients were lowered down the outside of the hospital on ropes after the fire broke out in the early hours of Friday at the privately-run AMRI hospital, engulfing the multi-storey premises in thick smoke. Firefighters and staff smashed glass windows to evacuate some of the 160 patients, with local media alleging that fire alarms and extinguishers had not been working. "In all, 90 bodies have been extricated from the hospital. 88 of these bodies have been identified and handed over to the relatives," Damayanti Sen, joint commissioner of police, told AFP on Saturday. Sen, who is heading the team appointed to investigate the tragedy, added that all the deaths were due to the inhalation of toxic fumes which quickly filled the wards. Four staff were among those killed. Initial investigations suggested the fire might have been started by a short circuit in the basement, which was used to store oxygen cylinders, plastic pipes, fibre coils, chemicals and medical equipment. Fire engines had trouble reaching the hospital, which is surrounded by narrow roads, and distraught relatives gathered outside during the rescue operation. "We heard the patients crying for help," said Shivani Mondal, 60, who lives in a slum settlement next to the hospital in the south of the city. "The fire brigade had not arrived by then, but the security guards and officials did not allow people from our locality to enter the premises to help them," he told AFP. "Finally young men used bamboo ladders to rescue who they could." Samir Nandi, a 50-year-old state railways employee, said the teeming city of 14 million people would be haunted by the tragedy and the failures that caused the death toll to be so high. "I have never seen such devastation before," said Nandi. "This incident shows that our hospitals are no better than cremation grounds." Javed Khan, head of the fire service in West Bengal state -- of which Kolkata is the capital -- suggested the incident pointed towards gross negligence and serious violations of safety norms. "There was a fire in 2008 in the same hospital and we are trying to probe how the authorities got their fire licence renewed," Khan told AFP. Subrata Mukherjee, a state minister, on Friday said senior members of the hospital staff appeared to have fled as soon as the fire broke out, abandoning patients, many of whom were sedated, elderly or infirm. "It was horrifying that the hospital authorities did not make any effort to rescue trapped patients," Mukherjee told reporters. Six senior executives from the two companies that co-owned the hospital were due to appear in court later Saturday. West Bengal Chief Minister Mamata Banerjee, who visited the site as relief work was underway, said they could face charges of culpable homicide. Prime Minister Manmohan Singh voiced his "shock and anguish" at the heavy loss of life.