In the October case, homeless men were rounded up at Sendai’s train station by Sasa, then put to work clearing radioactive soil and debris in Fukushima City for less than minimum wage, according to police and accounts of those involved. The men reported up through a chain of three other companies to Obayashi, Japan’s second-largest construction company.

Obayashi, which is one of more than 20 major contractors involved in government-funded radiation removal projects, has not been accused of any wrongdoing. But the spate of arrests has shown that members of Japan’s three largest criminal syndicates – Yamaguchi-gumi, Sumiyoshi-kai and Inagawa-kai – had set up black-market recruiting agencies under Obayashi.

“We are taking it very seriously that these incidents keep happening one after another,” said Junichi Ichikawa, a spokesman for Obayashi. He said the company tightened its scrutiny of its lower-tier subcontractors in order to shut out gangsters, known as the yakuza. “There were elements of what we had been doing that did not go far enough.”