Police in Japan have uncovered a ring of 17 men believed to have filmed 10,000 women in hot springs.

The men hid in mountainous areas near the springs and used telephoto lenses to film the women.

They also held gatherings to view the footage together, police say.

The group’s ringleader, Karin Saito, 50, was arrested in December 2021, per the Yomiuri Shimbun. Saito was nabbed in the Hyogo Prefecture, west of Kyoto, and charged under a nuisance prevention ordinance, a local law against illicit photography, The Asahi Shimbun reported.

Between December 2021 and February, 16 more men were arrested, including a doctor from Tokyo, senior company executives, and local government officials, reported the South China Morning Post. The men were detained by the police in 11 different prefectures, per The Asahi Shimbun.

Saito has confessed to taking voyeuristic photos of nude women for 30 years in 46 different prefectures, the police told the Yomiuri Shimbun. After being arrested, Saito informed the police about at least a dozen others in his group, reported the SCMP.

Saito and his crew used high-end camera equipment like long-focus telephoto lenses to film women bathing in the open-air hot springs, the Japanese police told The Asahi Shimbun. The men would take these pictures while hiding in mountainous areas several hundred meters from the springs, the police said.

The group would also get tips from Saito on how best to photograph the women, and hold gatherings to view the footage together.

Yutaka Seki, an executive director at the Japan Hot Springs Association, told the SCMP that while photography and filming are prohibited in hot springs, new technology makes it difficult to fully prevent such cases from happening.

“I am very much in favor of both sexes being able to bathe together in a friendly and safe manner, but incidents such as this attract bad publicity and worry people,” Seki told the SCMP. “And that makes my hope of mixed, communal bathing — as used to be the case in the past — more difficult.”

In 2018, people convicted of taking illicit photographs faced a year in prison and a fine of a million Japanese yen ($8,784), per the SCMP.

Hot springs, or “onsens” as they are known in Japanese, are popular across the country amongst locals and tourists. Known for their mineral-rich hot water, these baths are said to have a relaxing impact on the body and mind and are meant to be enjoyed naked.