There is no law which requires an employer to verify past employment. Many employers will refuse to do so, especially when they know that they had unlawfully employed the employee to begin with. When an employer refuses to provide a letter confirming employment it is sometimes an option to obtain an affidavit from a co-worker employed at the same time was you who has personal knowledge of your prior employment and duties performed. Consult with an experienced immigration attorney for case specific advice.
Jeffrey A. Devore, Esq.
Board Certified Immigration Attorney
Devore Law Group, P.A.
2925 PGA Blvd., Suite 204
Palm Beach Gardens, FL 33410
Telephone: (561) 478-5353
Facsimile: (561) 478-2144
No, no law can require that they write a letter.
What kind of visa did you have when you worked for them?
Perhaps there is something in the visa papers that may help.
FORMER IMMIGRATION LAW PROFESSOR -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.