Those factories, which are hired by middlemen to produce garments for fashion brands, paid their sewers as little as $2.77 an hour, according to a person familiar with the investigation.

The Labor Department declined to comment on the details of the investigations. In a statement, a spokeswoman said the department “continues to ensure employers receive compliance assistance with the overtime and minimum wage requirements, and the Wage and Hour Division is committed to enforcing the law.”

After repeated violations were found at factories making Fashion Nova clothes, federal officials met with company representatives. “We have already had a highly productive and positive meeting with the Department of Labor in which we discussed our ongoing commitment to ensuring that all workers involved with the Fashion Nova brand are appropriately compensated for the work they do,” Erica Meierhans, Fashion Nova’s general counsel, said in a statement to The Times. “Any suggestion that Fashion Nova is responsible for underpaying anyone working on our brand is categorically false.”

In 2018, Mr. Saghian said about 80 percent of the brand’s clothes were made in the United States. Fashion Nova’s supply chain has shifted since then, and now the brand says it makes less than half of its clothes in Los Angeles. It would not specify the overall percentage made in the United States.

The company does not deal directly with factories. Instead, it places bulk orders with companies that design the clothes and then ship fabric to separately owned sewing contractors, where workers stitch the clothes together and stick Fashion Nova’s label on them.

The brand’s clingy dresses and animal-print jumpsuits are often made by people like Mercedes Cortes, working in ramshackle buildings that smell like bathrooms.

Ms. Cortes, 56, sewed Fashion Nova clothes for several months at Coco Love, a dusty factory close to Fashion Nova’s offices in Vernon, Calif. “There were cockroaches. There were rats,” she said. “The conditions weren’t good.”