When you apply for U.S. citizenship ("naturalization"), U.S. Citizenship and Immigration Services (USCIS) is going to check your criminal background and make sure you are not a threat to the security of the United States. USCIS collects your fingerprints and asks other government agencies, such as the Federal Bureau of Investigations (FBI), whether they have any information about anyone matching your fingerprints.
Even if you're over 75 and didn't have to pay a fee for it, USCIS will ask you to provide "biometrics," including fingerprint records, as part of its background check process on all applicants for naturalization. Currently, the only other biometrics that USCIS gathers are photographs of you, plus your signature.
A few weeks after it gets your application for naturalization, USCIS will send you a notice in the mail asking you to appear for fingerprinting at one of its application support centers; most likely the one closest to where you live. If you don't show up for your biometrics appointment without good cause and without notifying USCIS, your application for naturalization is considered abandoned and you will have to reapply.
All USCIS application support center facilities are accessible to applicants with disabilities. If you are unable to attend any biometrics appointment because of a disability, or if you're in the hospital or unable to leave your house, USCIS will make special arrangements to take your biometrics.
You have to let USCIS know about your situation and request that someone from USCIS come to you for biometrics. To make the request, call the USCIS customer service at any time at 800-375-5283 (TDD: 800-767-1833). USCIS might ask for a copy of the appointment notice and medical documentation verifying the need for an in-home or in-hospital appointment.
USCIS won't need to fingerprint you if you are unable to provide fingerprints because of a medical condition, birth defect, physical deformity, skin condition, or psychiatric condition. A USCIS officer responsible for overseeing applicant fingerprinting will decide whether you can be fingerprinted or not.
That officer will have to meet with you in person and try to take your fingerprints first. The officer won't give up just because you have fewer than ten fingers, or the officer thinks your fingerprints will be "unclassifiable" (unable to be read clearly), or your condition is temporary.
If you can't be fingerprinted, you'll have to provide local police clearance letters from everywhere you've lived during the period of good moral character that applies to you. (Usually this is the five years before you applied, or three years if you're applying on the basis of marriage to a U.S. citizen.) You will also need to give a statement under oath about any criminal background you have in the period of good moral character.
Once it has your biometrics, USCIS will submit the records to the Federal Bureau of Investigation (FBI) for a full criminal background check. The FBI has a National Name Check Program (NNCP) that does a search for your name to see whether it appears in any personnel, administrative, applicant, or criminal files compiled for law enforcement purposes.
You can't have your naturalization interview with USCIS until the FBI name check is complete and you've been cleared.
The FBI will determine either that you have no administrative or criminal record, that you do have such a record, or that it doesn't know because it can't read your fingerprints.
The results of the name check are considered good for 15 months. If USCIS hasn't given you citizenship by that time, you will have to wait for the FBI to do another name check.