By Habib Beary in Bangalore
The Kaiga plant is highly protected (Photo: K Venkatesh)
A "disgruntled" worker could be behind the leak of a radioactive substance into drinking water at an atomic power plant in southern India, police say.
Preliminary investigations suggested it was an "inside job", a senior police officer told the BBC.
Police have moved into the Kaiga plant on the west coast of India, 450km (280 miles) from the city of Bangalore.
Fifty-five workers needed medical help for exposure to radiation after tritium contaminated a water cooler.
The Kaiga plant is highly protected and outsiders have little access to it.
"Both central and state agencies are investigating the matter. A list of people who were on duty on the day the incident took place has been given to the investigators," plant director JP Gupta said.
Inspector general of police Gopal Hosur told the BBC that there was no terror link to the incident.
"If that was the case the magnitude would have been bigger."
He said preliminary inquiries suggested that it was "an inside job".
Officials suspect that an employee had mixed the radioactive substance into a drinking water cooler meant for staff.
Chairman of the Indian Atomic Energy Commission Anil Kakodkar has called it a "malevolent act".
Although officials say the leak poses no risk to public safety, there is an element of panic in and around Kaiga.
Tritium, also known as Hydrogen-3, is used in research, fusion reactors and neutron generators.