'We have already informed the district police and administrative authorities about it,' Mohapatra said.

NALCO is Asia's largest integrated aluminium producer. The mine, which was shut after the attack, re-started work April 22.

The company has recently augmented security at most vulnerable points, including at places where the explosives are stored. The number of security personnel deployed at the mine has been increased from 89 to 197, the official said.

The company has also drastically reduced the quantity of explosives stored. As per the latest measures, the explosive stock is kept to the bare minimum, enough for about 4-5 days' consumption, the official said.