Thousands of Co-op City residents are up in arms after being slapped with bill for up to $10,000 due immediately.

Building management company River Bay's office was packed Monday with residents demanding answers.

River Bay executives told News 12 off camera that they're the middlemen. They say the issue comes from the state. Because Co-op City offers affordable housing, residents must fill out an income affidavit to show they qualify. They list Social Security numbers for all residents, including people who do not have incomes.

The state's Department of Homes and Community Renewal required River Bay to send out a notice directing residents to get a letter certifying their income. River Bay says 7,000 residents got the letter, which is nearly half of Co-op City residents.

While some residents didn't complete the certification, others did but are still waiting for the Tax Department to respond.

The state's review is also years behind, requiring residents to get documents from six years ago.

River Bay said in a statement, "Residents who, after notice, fail to provide certified tax returns, are charged the maximum annual surcharge."

River Bay told News 12 that affected residents can still fill out the state tax form and submit the letter they receive to River Bay to remove the fee if they are eligible.