The illness saw people suddenly falling unconscious after suffering from fits and nausea
Lead and nickel have been found in 25 of 45 samples from people who were among hundreds to have contracted the mystery illness in Andhra Pradesh's Eluru.
The samples, tested at Delhi's All India Institute of Medical Education and laboratories of other top institutes, included blood, urine and stool, as well as milk, rice, dal, vegetables and even soil.
On Tuesday four of five samples tested by AIIMS had reported positive for the heavy metals.
Nearly 600 cases have been reported so far, although health authorities are relieved because the increase in new cases has slowed; only 13 were reported till late Wednesday afternoon. In addition, only 70 of the reported cases are still in hospital. The rest have been discharged.
Doctors, however, still do not know what the illness is.
Most people reported a single episode of convulsions, fainting, temporary loss of memory, and nausea. There were a few repeat episodes, K Bhaskar Rao, the Andhra Pradesh Health Commissioner, told NDTV.
The state government has completed two rounds of household surveys to check for more cases.
Mr Rao said no abnormalities (or lead or pesticides) had been found in either water or air samples, and that other probable means of transmission - fruits, vegetables, soil - were being tested.
Investigators are also trying to identify possible sources of lead and organochlorine. They have also not ruled out pathogens, though viral and bacterial cultures were not found.
"This appears to be a case of acute and not chronic exposure. Lead levels, for instance, seem to go down a few hours after the episode (when samples were taken)," Mr Rao said.
So far, the cases seem to be limited to places in and around Eluru and include even people who consume mineral water regularly.
As of now, all that is known about the illness is that it appears to be mild and is not communicable. Only one case presented with epileptic seizure and repeat episodes are rare.
The illness is not limited to the Eluru municipal area; cases have been reported from the Eluru rural and Denduluru regions also, where the municipal corporation does not supply the water.
No more than one or two persons are affected per family and only a few houses are affected in each locality. The illness affects people across age groups and gender.
A two-member team from the World Health Organisation (WHO) is in Eluru to help with the investigation.
Chief Minister Jaganmohan Reddy has asked public health personnel and other departments to "probe the issue deeply and find out reasons for the illness," a statement from his office said.
Mr Reddy - who visited the hospital in Eluru on Monday, where 150-odd patients were admitted - also met Governor Biswa Bhusan Harichandan to discuss the situation.
On the Governor's recommendation, teams have been flown in from AIIMS, the National Institute of Nutrition, the Indian Institute of Chemical Technology (IICT) and the Centre for Cellular and Molecular Biology.