TULSA, Okla. -- Tulsa-based QuikTrip Corp. has agreed to pay nearly $750,000 in overtime back wages to thousands of employees in nine states. More than 3,800 current and former employees are affected.

An investigation by the U.S. Department of Labor found that the company failed to pay the additional overtime premium due on performance-related bonuses.

The bonuses stemmed from a "mystery shopper" program the company runs, where a QuikTrip official inspects a store at random to ensure it is being run correctly, store shelves are properly stocked and store operators are acting cordially toward customers. The "mystery shopper" program is one bonus program the company offers besides an employee's hourly wage.

Company spokesman Mike Thornbrugh said a computer glitch failed to give some employees the proper amount of bonus credit they earned from that program.

Thornbrugh did not know what triggered the investigation, but said Monday that "we're working in a cooperative fashion," with the Labor Department. "It's not adversarial."

The states involved are Arizona, Georgia, Illinois, Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas.

While an employer is not required to provide a bonus, if a nondiscretionary bonus is paid, it must be included as part of the employee's regular rate of pay for purposes of computing overtime, the department said.

The Fair Labor Standards Act, which QuikTrip was accused of violating, requires that covered employees be paid at least the federal minimum wage and receive overtime at one and one half times their regular rate of pay for hours worked beyond 40 a week, the department said.

Last week, the minimum wage went to $7.25 per hour.

"I am pleased that this case has resulted in almost $750,000 in back wages being paid to thousands of workers across nine states," Secretary of Labor Hilda L. Solis said in a statement Monday. "I am committed to ensuring that every worker is paid the full wages he or she is due, and that those who work overtime receive the compensation to which they are legally entitled."

QuikTrip is a privately held regional chain that operates hundreds of convenience stores.