Two men were questioned by the police in Hong Kong Friday in connection with a plot to rig big money races by shooting poison darts at horses.

The men were arrested Thursday evening after being seen acting suspiciously by security guards on a public jogging track that goes around the Happy Valley racecourse.

A police source quoted by government-run radio station RTHK said the men were questioned over a suspected plot to affect the outcome of races by shooting poison darts at horses from remote-controlled devices hidden in the grass next to the track.

In March 2007, a firing contraption attached to a series of tubes dug into the ground was found near the starting line of the racecourse, where millions of US dollars are staked by gamblers on weekly horse races.

Police officials said at the time they believed the discovery foiled a plot to fire poison darts from the tubes at the horses. Four men were seen acting suspiciously around the course in the days before the contraption was found, but no one was arrested.

The device consisted of a 12-metre-long polyethylene hose connected to an electrical device, a compressed gas cylinder and a camera lens. Twelve sets of two to three metal tubes were connected to the hose through sets of valves.

The two suspects arrested Thursday evening were carrying torches and gloves. A thorough search of the area was conducted after the men's arrest, but the police did not say if anything was found.

The Happy Valley racecourse is open to the public when races are not being run. It is patrolled by guards and monitored by closed-circuit television, and security has been stepped up since the 2007 incident.

The Hong Kong Jockey Club, which operates horse racing in the former British colony, offered a $128,000 reward for information leading to the conviction of the culprits, saying the plot could have seriously injured a horse or rider had it not been foiled.