When the IRS discovered that Juan was using someone else's SSN, you would think that he would have been arrested. Instead, he received the letter below giving him an ITIN (Individual Tax payer Identification Number) to use until he could straighten out the "mistake."
In the same letter, the next page tells Juan that he really doesn't have to go the the SS Agency to "straighten out" his SSN, because he has a privacy right not to give out his SSN which he doesn't have in the first place (see disclaimer below).
This says: "The Privacy Act of 1974 requires us to inform you that disclosure of your Social Security number is not mandatory. However, without your Social Security number we may be unable to correct your record. This could effect your future eligibility for benefits and the amount of benefits to which you may become entitled. The information you furnish will only be used to correct your Social Security record."
==========================================================================
This bizarre advisory pretends that the individual just made an honest mistake.
Without asking for it, Juan received his ITIN. The double sided letter offering help, has Juan's new ITIN on the Spanish side.
New ITIN
So why doesn't the IRS report illegals using ITINs to the ICE? Because the information contained in the IRS database, is by law, not allowed to be shared with any other federal agency unless it is subpoenaed.

It gets even crazier:

All ITINs start with the number nine. It's a dead giveaway to any employer who is provided with an ITIN with the first digit "9" in place of a Social Security number, that the would-be employee is not authorized to work in the U.S.

Not only can Juan use his ITIN to file his taxes, his "dependents" can also obtain ITINs from the U.S. Consulate in Mexico (thanks to a tax treaty between the U.S. and Mexico). Moreover, the IRS has no way to verify that the family member is actually dependent and receiving support from the illegal alien tax payer.

The above document is outdated, but nothing has change except that the illegal alien must now fill out an ITIN application W-7 form which can be processed at any IRS office or handled by mail.  In order for the IRS to approve the W-7, the first-time alien filer must oxymoronically prove that he has a need to pay taxes (actually a need for a refund) earned at a job he's not supposed to have in the first place, by presenting a completed tax return with W2s (with fake SSNs), but with the return's SSN field left blank (note that the W-7 instructions lists two out of 13 forms of ID required for a W-7. The two that aliens use are the matricula and birth certificate -- both forgeable). The return is then forwarded along with the W-7 application to the IRS's special ITIN Unit in Austin, Texas. The IRS then assigns an ITIN to the submitted return and processes it. After about eight weeks the illegal receives in the mail, the confirmation of his filed tax return along with his refund and his ITIN number. Thereafter, the illegal files his return like everyone else, except he used an ITIN instead of a SSN (but the attached W2/s still have fake SSNs).

This IRS ITIN revision states in the fifth paragraph: "Earlier this year, the IRS issued letters to all governors and state motor vehicle departments advising that ITINs were not designed to serve as personal identification and would not be suitable for determining identification of applicants for driver’s licenses."

In spite of the IRS's disclaimer, 13 states presently accept the ITIN in place of a SSN to obtain a valid driver's license even though the revision


This disclaimer was rubber stamped here
This says "the return filed with us for the account identified above shows your social security number as 555-10-3033. Our records show this number is assigned to another taxpayer."