A nonpartisan government watchdog group Monday announced it will push for reforms to the Cook County assessor’s office, citing Chicago Tribune/ProPublica Illinois findings that call into question the accuracy and fairness of the county’s property tax assessment system.

The Illinois Campaign for Political Reform, a nonprofit advocacy group, called for oversight of the assessor’s office, an explanation from Assessor Joseph Berrios of the methods his office used to value property and a plan to address inequities.

If the assessor’s office fails to take those steps, the group said the county should create an independent board to increase transparency and improve fairness and accuracy.

“We think these are some pretty basic measures that are absolutely necessary for the assessor’s office to engage in,” ICPR Executive Director Sarah Brune said in an interview.

In urging change, the group cited reporting from “The Tax Divide” series, which launched in the Tribune in June and has continued this month in partnership with ProPublica Illinois.

The series showed the assessor’s office overvalued low-priced residential properties while undervaluing high-priced ones. These disparities in assessments — known as regressivity — led to inequities in property tax bills, giving the wealthy unsanctioned tax breaks while punishing low-income residents.

The series found similar regressivity in commercial and industrial assessments, as well as tens of thousands of identical valuations over multiple reassessment periods, each three years apart. Experts said repeating values would be virtually impossible had Berrios done his job thoroughly.

County residents have been left “wondering whether they have been treated unfairly, and why these issues were allowed to compound over many years with little oversight or accountability,” the group said.

The ICPR said that of the four largest property tax jurisdictions in the country — including Los Angeles County, California; Harris County, Texas; and Maricopa County, Arizona — only Cook County fails to “conduct studies to ensure sufficiently equitable property assessments.”

Deputy Assessor for Communications Tom Shaer said the ICPR’s proposal relies on findings by the Tribune and ProPublica Illinois that the office questions, though the assessor has not provided evidence to support its criticisms. Shaer also said that Berrios’ office has been working to improve the assessment system.

“Assessor Berrios has been proactive in addressing problems in the system which he did not create,” Shaer wrote in a statement.

The latest in a growing chorus of groups calling for changes in the county’s assessment system, ICPR made its call for change less than a week after three public interest law firms sued Berrios and the county in Cook County Circuit Court. The lawsuit alleged violations of state and federal civil rights and housing laws.

The lawsuit seeks many of the same reforms pushed by the ICPR, including disclosure of the assessor’s methods and data, independent oversight of the office and new measures to ensure fair and accurate assessments.

The assessor’s office was already under scrutiny from the county’s inspector general, who was forced to take Berrios all the way to the Illinois Supreme Court to gain access to records needed to conduct investigations of the office.

In July, the County Board summoned Berrios to testify about the office’s practices, and Board President Toni Preckwinkle ordered a study of how residential assessments are done.

Shaer said Berrios is open to scrutiny and change.

“He is cooperating fully with the current study of property assessment being conducted for Cook County by the Civic Consulting Alliance,” he wrote in a statement. “The Assessor has committed to implementing any changes and improvements possible which are recommended by the CCA study.”

Brune said her group plans to reach out to county commissioners and other community leaders to ensure changes to the assessor’s office are made.

“We want to make this a priority going forward,” she said.