Investigators on Monday questioned employees present in the operating area of the Kaiga Nuclear Power Plant after preliminary findings revealed "internal sabotage" for the radioactive contamination of drinking water at the high-security complex. The probe team,which includes nuclear scientists,questioned the employees present on the night intervening November 23-24 during which radioactive heavy water (tritium) vials were put in the water cooler,said a senior official in the plant in Karnataka's Uttara Kannada District. "Relevant agencies are conducting the probe. Questioning is part of the investigation," he said. In New Delhi,Union Minister Prithviraj Chavan said the Government was taking the issue "very seriously" as it was "breach of some security measures". Chavan,who is the Minister of State for Atomic Energy,said,"All agencies are looking into the matter. Somebody from the lab,who had access to the water cooler,had done it between 3 am and 6 am (of November 24)." On the medical treatment provided to around 50 employees who were exposed to radiation after drinking the cooler water,Chavan said,"All the people have been medicated and they have returned to work." Kaiga plant officials said the computer access control system has a record of all the personnel who had entered the - operating island - as the contamination incident is believed to be an insider's job. Terming the incident as a "malevolent act",Atomic Energy Commission Chairman Anil Kakodkar has said,"Somebody deliberately put the tritiated water vials into the drinking water cooler". Officials said that preliminary probe has shown it was an act of mischief and did not reveal any violation of operating procedures or leak. Nuclear Power Corporation's Chairman S K Jain said all the units of the plant were normal and none of its workers has been admitted to a hospital. Kaiga plant Station Director of the Kaiga plant J P Gupta said today,"There is no radioactivity change in the local environment. "There is no release of radioactivity to the environment within the plant site and outside," Gupta said. "All plant systems are functioning normally. There is no cause whatsoever for any radiation safety concern." Chavan said,"What I have inquired from Kakodkar and Jain is that somebody put tritiated heavy water in the water cooler. It was definitely done to hurt anybody. "The (human) system clear up in a day or two (in case of such contamination." The employees working in the first maintenance unit of the power station were treated at the plant hospital in Mallapur for increased level of radiation. According to B Bhattacharjee,Member,National Disaster Management Authority,the cooler was found to be sealed. "But later on,the investigators found that through the drainage,the overflow line of the cooler,that it seems some mischief monger inserted some tritiated water and contaminated the whole cooler." Kaiga currently has three 220 mw plants - two are operational and one has been shut-down from October 20 for annual maintenance. A fourth one is slated to be operational from later this financial year. Few years back a similar malevolent act was observed in Tarapur atomic power sub-station and the culprits were dismissed after investigations.