The Japan Racing Association says it has learned how a banned substance was mixed into dietary supplements for horses.
The association disqualified 156 horses from races last month after discovering they may have been fed a prohibited substance.
The association says 28 stables were sold dietary supplements containing theobromine, which can act as a stimulant.
JRA officials held a news conference on Friday and released findings based on its questioning of the manufacturer.
They said they learned that a powdered sub-product of cacao beans, which can contain theobromine, was being processed on an adjacent production line, and that the powder was scattered onto the supplement production line by accident.
They said the two lines were located about 20 meters apart.
The cacao sub-product is used for fodder for other livestock.
The officials also found that the manufacturer had been conducting factory inspections only once a year, in violation of JRA regulations, and that its checking system was ineffective.
The JRA has asked the company to ensure there is sufficient distance between production lines and to inspect one out of every 600 bags of supplement produced.
The organization also plans to pay the affected stables up to about 4.6 million dollars in compensation.
A JRA director, Masahiro Usuda, says the association will ask testing organizations and retailers to conduct thorough checks and provide information about any products that may contain banned substances.