Giant coca plants said to resist herbicides and yield eight times more cocaine may be due to extra fertilizer, not a drug cartel’s genetic modification program, a scientist said on Tuesday.

A Colombian police intelligence dossier quoted in the Financial Times said smugglers apparently received help from foreign scientists to develop a herbicide-resistant tree that yields eight times more cocaine than normal shrubs.

But a toxicologist who studied the plants for the police said he knew of no evidence that showed whether the plants were genetically modified or merely grew big because they received an unusually large amount of fertilizer.

“Up to now there is no scientific evidence, at least in our country, which shows this is the consequence of genetic manipulation,” said toxicologist Camilo Uribe. “They could simply be the result of an excess of fertilizer,” he said.

A few isolated giant plants had been found in areas including Colombia’s Sierra Nevada and Macarena mountains, he said.

The United States has provided more than $3 billion of mainly military aid to back a crop-spraying program that the Colombian government says has cut the country’s coca-growing area by almost two-thirds.

Washington dismissed media reports of genetically modified coca in August.

“We regularly hear rumors that narcotraffickers are working to create a transgenic form of coca, but there is no scientific proof that they have undertaken such research,” Phyllis Powers, Director of the Narcotics Affairs Section of the U.S. Embassy in Bogota, said at the time.