ST. PAUL — Minnesota’s attorney general sued a debt collector Wednesday over steep interest rates charged on overdrawn checking accounts at two big banks.

Attorney General Lori Swanson alleged that Minneapolis-based Bradstreet and Associates broke state law by charging up to 21.75 percent in interest on bank debt. The lawsuit involves $18 million in debt and unknown interest costs that the company was pursuing from people who overdrew Wells Fargo and US Bank accounts. Swanson isn’t alleging wrongdoing by the banks, which sold off the debt they figured they would have little luck recovering.

Swanson said the assessed interest rates weren’t allowed in customers’ original banking contracts and were many times what state law allows. The two-term Democrat said Bradstreet sometimes went to court to get default judgments that tacked on thousands of dollars to the original amount of bounced checks.

“Companies have the right to collect legitimate debt. But they shouldn’t be charging people for interest they don’t owe, and they shouldn’t be getting court judgments against people for interest they don’t owe either,” Swanson said at a Capitol news conference.

Swanson said she believes as many as 16,000 Minnesota consumers could be affected. The case filed in Hennepin County District Court seeks restitution and a halt to the collection practices.

Calls by The Associated Press to Bradstreet’s phone numbers on Wednesday did not go through, and the company did not immediately return an email seeking comment. Bradstreet is a “debt buyer,” which typically pay pennies on the dollar to buy bundled debt under the expectation it can turn a profit tracking money down. It obtained the bank debt through a separate company that has purchased billions in debt from large national banks. Swanson previously sued a Florida company that was the intermediary.

Swanson said the debt-flipping pattern concerns her and she is looking deeper into it.

Nicole Garrison-Sprenger, corporate relations vice president for U.S. Bank, said it is fully cooperating with Swanson’s investigation into the debt it sold. She noted the bank isn’t party to the case.

“We support the attorney general’s efforts to crack down on fraudulent debt collection practices,” she said.

A Wells Fargo spokeswoman declined to comment.

Jennifer Dahlke, a 42-year-old veterans hospital social worker from Minneapolis, said she felt scammed by Bradstreet and intimidated by its aggressive attempts to collect debt on $2,073 in bank overdrafts. She said Bradstreet took her to court to demand she pay $4,311 and even called her elderly parents to ask them to cover the debts.

“They were using the courtroom basically to take more money out of my pocket,” Dahlke said at news conference in the attorney general’s office. “I felt very helpless and very hopeless and alone. I didn’t know what I was up against.”