Indian authorities were investigating possible negligence after 18 hospital patients died when rainwaters from massive floods in southern Tamil Nadu state knocked out generators and switched off ventilators.

The patients were in the intensive care unit at MIOT International hospital in the state capital of Chennai when floodwaters seeped into the room with the generators, cutting off power to the building and the ventilators earlier this week, state Health Secretary J. Radhakrishnan said Saturday.

The worst flooding in a century in Tamil Nadu has left more than 280 people dead since November. In the latest deluge this week, authorities turned off power in some areas to prevent electrocutions that were blamed for several deaths.

Although floodwaters have begun to recede, vast swaths of Chennai and neighboring districts were still under 2 1/2 to 3 meters (8 to 10 feet) of water, with tens of thousands of people in state-run relief camps.

Army soldiers using boats have rescued thousands of residents marooned in high-rise buildings and launched massive relief operations to provide food and medicine.

"We feel quite helpless," said Malti Soman, standing in knee-deep water in Mambalam, a residential area in central Chennai. "The landline phones are not working. And my cellphone is dead because there is no electricity to charge it."

Radhakrishnan, the health secretary, said that while the immediate rescue operations were tapering off, the main focus in the coming days will be to prevent the spread of communicable diseases.

In many areas, sewage drains have overflowed, posing a health hazard for residents who have had to wade through the water, Radhakrishnan said.

"This is a concern," he said. "We are working with the local water and sewage agencies to disinfect the worst-hit areas."

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Sanitation workers have begun spraying insecticide in many places to prevent the spread of vector-borne diseases such as malaria and dengue due to the stagnant water collected in large parts of the city.

Chennai's airport was closed for a fourth day Saturday, although some flights operated from a nearby air force base.

Railway services resumed partially, with a few trains running from Chennai's main train station.

India's main monsoon season runs from June through September, but for Chennai and the rest of the southeastern coast, the heaviest rainfall is from October to December, also called the retreating monsoon. Experts say the heavy rainfall was linked to the El Nino weather pattern, when the waters of the Pacific Ocean get warmer than usual.