TAIPEI (Taiwan News) — The National Immigration Agency arrested an illegal broker accused of managing 37 unaccounted-for workers, most from Thailand, reports said Wednesday.

The case began after a tip last year that a private apartment in Taichung was being used to house unaccounted-for foreign workers. After months of surveillance and investigation, a special task force identified a broker surnamed Cheng (鄭) as the operator, per CNA.

The agency said Cheng would show up near the apartment early in the morning in a van, then pack in more than 10 people and drive them at high speeds to work sites.

When the task force moved in, officers found 13 overstayers in Cheng’s vehicle. Searches of residences linked to him uncovered another 24 people, most of them Thai nationals.

Authorities estimated Cheng made about NT$5 million (US$160,000) a year. Each worker paid him NT$400 per day in illegal brokerage fees, the agency said.

Investigators said Cheng had previously been convicted in a similar case. Taichung City Government had fined him NT$600,000 for violating the Employment Service Act. In the latest case, the Taichung District Court sentenced him to four months in prison.