NEW DELHI, India (AP) — Health officials and experts are still baffled by a mysterious illness that has left over 500 people hospitalised and one person dead in the southern Indian state of Andhra Pradesh.

The illness was first detected Saturday evening in Eluru, an ancient city famous for its hand-woven products. People started convulsing without any warning, said Geeta Prasadini, the director of public health.

Since then, symptoms ranging from nausea and anxiety to loss of consciousness have been reported in 546 patients admitted to hospitals. Many have recovered and returned home, while 148 are still being treated, said Dasari Nagarjuna, a government spokesperson.

Teams of experts have arrived at the city from India’s top scientific institutes. Different theories have been suggested and are being tested. The most recent hypothesis is contamination of food by pesticides.

“But nobody knows,” Prasadini admitted.

What is confounding experts is that there doesn’t seem to be any common link among the hundreds of people who have fallen sick. All of the patients have tested negative for COVID-19 and other viral diseases such as dengue, chikungunya or herpes. They aren’t related to each other. They don’t all live in the same area. They’re from different age groups, including about 70 children, but very few are elderly.

Initially, contaminated water was suspected. But the chief minister’s office confirmed that people who don’t use the municipal water supply have also fallen ill, and that initial tests of water samples didn’t reveal any harmful chemicals.

A 45-year-old man with the single name Sridhar was hospitalised with symptoms resembling epilepsy and died Sunday evening, doctors said. Prasadini said his autopsy didn’t shed any light on the cause of death.

The hypothesis currently being tested is that people ate vegetables tainted with pesticides made of organic compounds containing phosphorus. But this is an “assumption” based on the fact that such pesticides are commonly used in the area and not on any evidence, Prasadini said.

She said that experts were testing to see if pesticides had contaminated fish ponds or spilled over to vegetables.

Opposition Leader N Chandrababu Naidu demanded on Twitter an “impartial, full-fledged inquiry into the incident”.

Andhra Pradesh state is among those worst hit by COVID-19, with over 800,000 detected cases. The health system in the state, like the rest of India, has been frayed by the virus.