Following Reuters’ first story on child labor at SMART last July, as many as 10 Hyundai suppliers in Alabama have been under investigation by state or federal authorities for child labor violations, Reuters reported in December.

One of the plants was inspected last August and authorities found and removed several children from the factory floor, later issuing penalties to the plant operator and a third-party staffing firm who recruited them.

“The use of underage labor at a supplier or any operation is unacceptable, and we are committed to making sure non-compliance never happens again,” Chang wrote in the letter. “This is a zero tolerance issue.”

As Reuters reported earlier, many of the underage workers who found their way into the Alabama auto parts plants were recruited by third-party staffing agencies, a process that can allow big corporations to turn a blind eye to the illegal employment of minors.

In the shareholder letter, Chang reiterated that Hyundai was “discouraging” suppliers from relying on such staffing agencies in the future.

He wrote that staffing firms who hired children to work at Hyundai supplier plants had provided false employee documentation. In the future, however, Hyundai and its supply chain partners must do more to ensure children are never put to work in their factories, the letter said.

“Ultimately, the responsibility is with Hyundai to make sure all our suppliers understand and meet our high global workforce standards,” Chang wrote.

U.S. and Alabama law prohibit people under age 16 from working in industrial factory settings, and anyone under 18 is prohibited from working in particularly dangerous roles in automotive plants, such as driving forklifts or operating metal-cutting and stamping machines.

Earlier this month, thirty-three members of Congress urged DOL to seek strong and swift penalties against those responsible for child labor in the Hyundai supply chain.