Superintendent of Police of Uttara Kannada district, Raman Gupta said, "We filed the FIR last night."
He said the FIR has not named anybody and is general in nature based on the complaint filed by Kaiga plant authorities.
According to police sources, all angles of the case, including probability of theft of tritium, which carries an astronomical price tag, is being investigated.
The investigation is being done by Intelligence Bureau and some officials of Nuclear Power Corporation of India (NPCIL), in coordination with the state police.
On Nov 24, some routine urine samples in workers indicated higher content of tritium than normal and the source of contamination was identified to be a drinking water cooler.
All except two of the about 50 workers, who had consumed contaminated water, have resumed their duties in the non-radiation area of the plant, NPCIL said.