The Treasury Department's Office of Inspector General said Tuesday that the IRS has sent out thousands of personal ID numbers to taxpayers who have died, and warned that these letters are only making it worse for the families of the deceased.

"Sending unnecessary IRS notices to surviving spouses and grieving families increases their burden," the OIG advised the IRS.

The IRS, run by Commissioner John Koskinen, has been sending out personal ID numbers to thousands of dead people. Chip Somodevilla/Getty Images

Since 2011, the IRS has handed out "identity protection personal identification numbers" to people who have been victims of tax identity fraud. The PINs are used by the IRS to verify that the electronic tax returns filed by these people are valid.

But because of a programming error, 13,220 of these PINs went to dead people in the last tax year, a practice the OIG said should stop.

"Not all taxpayers whose tax accounts have an identity theft indicator will receive an IP PIN," the report said. "For example, the IRS should not send an IP PIN to those individuals who are deceased or for whom mail was previously returned undeliverable."

"We continue to believe that the Customer Account Services Director should establish a process to ensure that IP PIN criteria are accurately programmed so that eligible taxpayers receive an IP PIN and ineligible individuals do not," it added.

While the IRS sent out more than 13,000 to dead people, the IRS failed to send out nearly 560,000 living people who needed these identification numbers.