I've changed the practice area to "immigration." An alien number is not pertinent to criminal warrants from state courts.
Beware of posting someone's identifying information on a public website. It is not a good idea; That said your father can consult an AILA immigration attorney about doing a Freedom of Information Act (FOIA) request with the appropriate federal agency and that will be a possible way to obtain his A#. Alternatively he can look at his immigration documents relating to applications he may have filed in the past; the A# may or may not be may be on these immigration records.
AILA's Immigration Lawyer Search
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File for FOIA.
Contact immigration attorney Gintare Grigaite, Esq. at 201-471-7989, located in New York and New Jersey. Contact immigration attorney Gintare Grigaite, Esq. for a free consultation about new Immigration Reform policies. Answers on AVVO do not constitute legal advice and do not form attorney-client relationship. Always consult an attorney for a legal advice.