TOKYO (AP) — The head of the utility behind Japan’s nuclear crisis has acknowledged that hundreds of workers at the contaminated Fukushima Dai-ichi plant were mobilized through a murky hiring system.

Tokyo Electric Power Co. President Naomi Hirose attributed the hiring problem to high worker turnover at the worksite. He also told The Associated Press on Monday that TEPCO is working to fix the problem, which he attributed to an industry-wide hierarchical contract system.

In a recent TEPCO survey of some 2,400 contract workers, dozens said they were instructed to falsify affiliations, while some complained they never received a written contract.

Hirose said securing a workforce that can last through decades-long post-disaster operations would be critical to cleanup efforts.