Four Chinese nationals have been arrested on suspicion of smuggling about 700 kilograms of illegal stimulants from the United Arab Emirates, police and customs officials said Wednesday.

The amount of stimulants, with an estimated street value of ¥43.4 billion ($311 million), was the second largest ever confiscated at one time in Japan.

Suspicious powders were found in March in containers on a ship that arrived at the Port of Tokyo via China and were seized by Tokyo Customs. Police in Tokyo suspect the involvement of an international smuggling ring and are investigating the potential connection.

According to the police, the four Chinese suspects are between the ages of 35 and 51 and live in Tokyo, Yokohama and Kawasaki.

Two of them were arrested on suspicion of conspiring with others to import a large quantity of stimulants by hiding them as cargo in containers that came from Dubai and transited China before arriving in Tokyo. The cargo was unloaded on March 5.

Stimulants were found inside 175 boards, each about 3 centimeters thick, placed in seven containers. Tokyo Customs found the suspicious powders during an inspection.

The boards were eventually transported to a company's premises in Chiba Prefecture. The police tracked down the cargo, which resulted in the arrest of the four.

The largest-ever one-time confiscation of stimulants in Japan was in 2019 when about 1 ton was seized at a beach in Shizuoka Prefecture. Seven Chinese men were arrested and charged in connection with that case.