Colorado's Department of Motor Vehicles issued U.S. citizen driver's licenses to 524 non-citizens, including some illegal immigrants, by accident, Denver CBS reported on Friday.

The licenses look akin to those given to state residents. The DMV blamed a software problem at a contractor, who is working with the DMV to retrieve the licenses and issue the correct IDs.

The licenses were mailed to the applicants at the beginning of September. The contractor said it learned on Sept. 8 that the printing error occurred Aug. 8.

"What happened is there was a software update error on the part of the vendor," said Director of Dept. of Revenue Barbara Brohl.

The DMV said that drivers left its offices with correct temporary licenses, but later, the contractor later mailed the incorrect permanent licenses.

The recipients of the licenses were sent an email on Friday asking for the licenses back. The correct credential will be issued by the contractor, it said, along with a gift card to compensate the individuals affected.

Lawmakers in the state passed a law that took effect Aug. 1 that grants licenses to non-citizens to improve public safety by making sure that all drivers in the state, regardless of their status, are licensed. However, they are marked with a black strip that reads "Not Valid For Federal Identification, Voting or Public Benefit Purposes."

"If they don't send the licenses back we'll go into Plan B and we'll figure that out but we will be getting them all back," said Brohl.

After the DMV announced it would issue appointments to non-citizens to receive licenses, calls and online orders deluged its offices. Tens of thousands reportedly got in contact with the DMV to register for the new ID, apparently clogging the system. The first day it began offering appointments, July 1st, the DMV scheduled 823, more than 100 an hour during office hours.

"The worry that I have, now that I've learned about this, is people could show up to register and vote the same day and their driver's licenses don't have that indicator," said Secretary of State Scott Gessler.

Those in the country illegally or without a Social Security number are able to apply for the license if they prove they live in Colorado and pay taxes. Those in the country illegally must prove they have applied for citizenship.