The gang struck at the bank during the closing hours last evening and also took away about Rs four lakh cash, police said the incident came to light only late last night.
A special team has been formed by District Superintendent of Police Abhinav Kumar to nab the culprits.
The dacoits had entered the Venmani Primary Agricultural Cooperative Bank at Keezhavenmani village under the guise of customers when bank secretary Kamaraj and a jewel appraiser were accounting the pledged jewels.
They also attacked appraiser Ganapathy till he fainted and gagged his mouth with a tape and bound him to a chair.
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Later they looted jewels worth Rs.Three crore and cash and left the bank with the booty, locking the door from outside, police said.
Ganapathy who regained consciousness later managed to remove the tape and shouted for help. A passerby, who heard the alarm and found the door locked, informed Police.
Police broke open the door and found the secretary murdered and released Ganapathy, who has been hospitalised.