Gold, weighing about 11 kgs, was seized in five incidents from passengers who allegedly tried to smuggle it via Indira Gandhi International Airport (IGIA), official sources said.
The incident came to light when packets containing the yellow metal, covered and sealed at the instance of courts, were opened before a departmental committee as part of inventorisation proceedings. A test revealed the sealed material was not gold, they said.
An FIR under Section 409 for criminal breach of trust by public servant was registered against unknown persons on Friday at the IGIA police station as it was suspected to be an insider's job.
"It is a very serious matter. The department has approached police to find out the culprits behind the mischief. All possible assistance will be provided to the police," a senior customs official said.
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This is not the first time that such a case has come to light.
An FIR was filed last year too to probe similar instances of theft.
Theft of gold from five packets had been reported to the police on January 16, 20, April 30 and December 23, 2014, the latest FIR said.
The police has launched a probe. "We will be issuing notice to the custodian of the warehouse as part of investigation proceedings," a senior police official said.
Both police and customs officials have not ruled out the complicity of insiders.