A woman in a suburb of Columbus, Ohio, was sued twice, by two different creditors, over the same overdue student loan. Another person, in Illinois, was taken to court over a loan that had already been paid off. And hundreds of borrowers faced lawsuits over debts so old that they were no longer legally collectible.

The cases all involved the same debt collector: Transworld Systems.

Student loans have soared over the last decade, becoming the largest source of household debt outside of mortgages. The tide of rising defaults has also turned into a lucrative business, with companies collecting tens of millions of dollars through settlements, wage garnishments and other compelled payments.

Transworld Systems has been one of most prolific debt collectors, filing more than 38,000 lawsuits in the last three years on behalf of a single client, the National Collegiate Student Loan Trusts. But many of the cases were flawed, as the debt collector churned out mass-produced documentation based on scant verification, according to legal filings by a federal regulator and a New York Times analysis of court records from hundreds of cases.

In September, the regulator, the Consumer Financial Protection Bureau, accused National Collegiate and Transworld, in separate complaints, of using sloppy and illegal collections methods. Both parties agreed to settle and pay more than $21 million in penalties and refunds.