The passports belonged to eight horses which are owned by the racing syndicate Classic Bloodstock and stabled at Brian McNath's yard in Newmarket.
Earlier this month, the passports were reported missing by Mr McNath before they mysteriously re-appeared in the yard's tack room. Despite the re-appearance, the Jockey Club decided to continue with its investigation .
Horse passports give a description of a horse and a record of their immunisations. Although they are not proof of ownership they are needed for a horse to be allowed to race.
David Pipe of the Jockey Club, the regulatory body for horse racing, said the Club's security department was investigating the matter and confirmed that an interview last week with Mr McNath, from whose yard the passports disappeared, had been about the passports.
At the same time as the passports disappeared, Ron Dawson, a director of Classic Bloodstock, removed a number of fillies from the McNath's yard and placed them with another Newmarket-based trainer, Stuart Williams.
Classic Bloodstock is run by Dawson and his wife, Maureen Moorcroft. In April this year, they recruited Bob Champion as a non-executive director to train the company's string of eight National Hunt Horses.
Trainer McNath is married to the a niece of Classic Bloodstock director Dawson. Williams is married to Dawson's daughter.
Classic Bloodstock began in 1994 and owns 23 horses. According to the company's head office, the syndication was subscribed to by at least 6,500 shareholders, which raised more than pounds 3m.
Dawson launched Classic Bloodstock II in March 1995, but has extended its subscription period for a second time. Originally extended to July 31 1995, a spokesman at the company's office said it was now open for subscription until August 31 1995. Despite the fact that the offer has not closed, Dawson has already bought two horses for that company. Members of the public are sent glossy brochures persuading them to subscribe to the syndicate for as little as pounds 250 for 1,000 shares.
Companies House has fined Classic Bloodstock pounds 500 for late filing of its company report. This is now due to be filed by 27 August.