When the Russians, which included a woman, were found by local fishermen moving around in their coastal village of Idinthakarai on Monday, they alerted the police.
The foreigners were accompanied by two persons from Valliyoor near here and a car driver, hailing from Thiruvananthapuram.
The Russian nationals were questioned by Kudankulam police for nearly six hours, the official added.
In the past, Idinthakarai village had been the hub of anti-nuclear protests.
Kudankulam has two 1,000 MWe nuclear reactors built with Russian assistance and four more similar units are coming up at the same complex.
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