Gambian authorities began making arrests related to the case in mid-May and subsequently invited Britain's Serious Organised Crime Agency to provide forensic assistance, the source said.
West Africa is a major stop off point for drugs, often from South America heading to Europe, although officials say it is impossible to say exactly where this consignment was heading at this stage.
In March Yayha Jammeh, the Gambian president, said there would be "zero tolerance" for drug-trafficking after 11 senior officials, as well as the director of Gambia's national drug enforcement agency, were arrested as part of a probe linked to drugs.
The agency is now under Jammeh's direct control.
"I have zero tolerance for criminals. I have double zero tolerance for drugs," he told state radio.
Those arrested included Nigerians, Ghanaians, Venezuelans and Dutch, a source said without giving further details.