The U.S. Social Security Administration paid as much as $1.3 billion in disability insurance benefits to thousands of Americans ineligible for the program, the Government Accountability Office says.

The program that offers a social safety net to working Americans in case of physical or mental impairment sent payments to 36,000 ineligible receivers between December 2010 and January 2013. The government watchdog says these beneficiaries exceeded the $1,000 per month income limit on eligibility.

In one of the most egregious instances of apparent fraud, a physician claiming disability received $90,000 in overpayments over three years, as he earned an income of as much as $22,000 per month during the five-month waiting period required for establishing eligibility. Two other beneficiaries received payments of more than $20,000 thus far this fiscal year, even as they’ve held jobs during the waiting period.

In addition, other beneficiaries were paid in violation of rules prohibiting benefits following a nine-month trial period that allows a claimant to return to work while continuing to receive disability payments. In one such case, a woman was paid $74,000 after she’d worked for more than nine months, while a man received $57,000 in benefits for the same.

The Government Accountability Office says the overpayments represent just one percent of all disability claims, but acknowledged the estimate could be a low-balled number.

The Social Security Administration claims a greater than 99 percent accuracy rate in claims assessments but said it would investigate. "While our payment accuracy rates are very high, we recognize that even small payment errors cost taxpayers," a spokesperson told reporters. "We are planning to do an investigation and we will recoup any improper payments from beneficiaries. It is too soon to tell what caused these overpayments, but if we determine that fraud is involved, we will refer these cases to our Office of the Inspector General for investigation."

Below is a video report from CNN: