Police have arrested seven Chinese men on suspicion of possessing around a ton of stimulants seized at a beach in central Japan, investigative sources said Wednesday.

The record seizure for a single bust in Japan occurred in Minamiizu, Shizuoka Prefecture, earlier this week. The quantity of stimulants, with an estimated street value of around ¥60 billion ($560 million), was slightly less than the 1,138.6 kilograms of stimulants confiscated across the nation throughout last year, according to the National Police Agency.

The country’s second-largest single stimulant drug seizure was 597 kg, which was found on a boat anchored at a port in Naha, Okinawa Prefecture, in 2016.

The seven Chinese men in their 20s to 40s were arrested Monday and Tuesday as some of them were unloading the drugs from a boat. All of the men have denied the allegation, the sources said.

The police suspect the involvement of a large international smuggling ring and believe the stimulants may have been transferred to the boat from another vessel while at sea, the sources said.

Local residents had reported sightings of a suspicious boat over several years, prompting the police to mount a stakeout on the beach, the sources said.