Head Constable Vijay Prathap Singh allegedly opened fire from a 9 mm carbine, killing Assistant Sub-Inspector Ganesan and Head Constables Subburaj and Mohansingh at the Central Industrial Security Force (CISF) barracks in Kalpakkam township, about 10 km away from the nuclear complex, yesterday.
"He was in a very depressed state of mind for some time," C Vijaya Kumar, who led the probe team, told PTI, adding that the CISF jawan was also "obvioulsy" homesick.
Vijaya Kumar denied reports that he was "harassed" by his peers and superiors.
"If some were talking, Vijay Pratap Singh had the habit of assuming that they were only talking against him," he said.
Singh was posted at Kalpakkam nuclear complex in June this year. He has been arrested and remanded to judicial custody.
Police said the two injured CISF personnel were undergoing treatment at a hospital.