Borrowers were at greater risk of defaulting if their student loans were handled by the Department of Education's main loan contractor than if they had dropped out of college or failed to complete a single class, according to a startling new report released Monday.

The study from the Association of Community College Trustees provides an unusual look into why some people default on student loans. Using previously confidential data, researchers examined all students who borrowed from the federal government to attend community colleges in Iowa and whose loans came due between October 2010 and September 2011. Nearly 28 percent of those loans ended up in default by January 2015.

The report adds to a growing body of evidence that the Education Department is failing to properly police its loan contractors. Serious delinquencies in the largest student loan program are on the rise , while about one-fifth of all student borrowers are in default on their loans -- troubling trends at a time when the economy is growing and unemployment is falling.