Children as young as 13 were manning phone banks for a company that does market research and political surveys from call centers in seven states, federal investigators said Tuesday.

Children that young can't legally be employed except on farms.

The U.S. Department of Labor fined Orem, Utah-based Western Wats for hiring three 13-year-olds, and for working an additional 1,479 children more than three hours on a school day or more than eight hours on a weekend day, among other violations. Those children were all 14 or 15 years old.

The $550,000 penalty was among the highest of its kind ever assessed against a U.S. company, officials said.

More unusual was the sheer volume of children the company was hiring, said Lee Ann Dunbar, the Labor Department's district director for Utah, Montana and Wyoming.

Dunbar said she found it unusual that a company would employ children that young to make cold calls to adults.

"The company would have to explain that," she said.

Western Wats disputed the fine and said it would fight the child-labor allegations in Utah, Arizona, Idaho, Kansas, Nebraska, South Dakota and Wyoming.

"We do disagree with the DOL's finding and have appeal on several grounds," corporate counsel Stacey Jenkins said. "We won't be able to comment further."

Investigators found that Western Wats sometimes paid children less than the $7.25-an-hour federal minimum wage, although those violations appeared to be an oversight and the company agreed to pay about $5,000 in back wages, Dunbar said.

"We did not find that it was a willful violation," she said.

No child was employed against his or her will. Western Wats' need for workers fluctuates wildly, and the hiring of children may have been necessary for the company to meet its obligations.

During the school year, children under 16 can't work more than three hours on a school day or more than 18 hours a week. When school is not in session, they are limited to 40 hours a week.

Labor investigators can make unannounced visits to U.S. workplaces randomly or based on tips or complaints, interviewing employees and reviewing payroll records. They wouldn't immediately be able to confirm a worker's age, but can check records.

"It's hard to tell the age of an employee just by looking," she said.

Dunbar wouldn't say what led investigators to Western Wats.