The ProPublica Illinois and Chicago Tribune project “The Tax Divide” is the winner of the Richard H. Driehaus Foundation Award for Investigative Reporting. The award honors achievement in government-related reporting in the Midwest.

Led by reporter Jason Grotto, the series shed light on a complex, specialized issue, but one of deep concern for many Chicago residents: property taxes. For decades, Cook County residents had suspected their property taxes were based on inaccurate assessments, overvaluing many low-priced homes while undervaluing many higher priced ones. But few reporters had the technical and analytical ability to prove there were problems.

Grotto — who started the investigation as a Chicago Tribune staff writer and continued it after joining ProPublica Illinois — studied the system for two years, analyzing more than 100 million electronic records, and interviewing dozens of experts, attorneys and property owners. His tenacious reporting exposed widespread inequities and egregious errors in assessments that punished small businesses and poor homeowners, while giving the wealthy unsanctioned tax breaks and lining the pockets of politically connected tax attorneys. The first three stories published in the Chicago Tribune; a fourth piece of the series and more than a dozen additional follow-up articles were published as a ProPublica Illinois/Chicago Tribune partnership, with the contributions of ProPublica Illinois data reporter Sandhya Kambhampati and Chicago Tribune reporter Ray Long.

Since the series, Cook County’s broken assessment system has finally been held to account. Within weeks of publication of the first stories, the county’s inspector general launched an investigation of the assessor’s office. The Cook County board required Assessor Joseph Berrios to testify at a public hearing, and state and local lawmakers introduced legislation to limit campaign contributions to the assessor. Ultimately, in the Illinois primary election last month, Berrios lost his bid for reelection.