Most U.S. medical boards require direct-source verification of medical education. If you're an IMG, you can either travel back to your home country and provide the forms and request to your medical school; you can enlist the help of family and friends in your home country to help; or you might want to consider paying the FCVS to put together a profile of your core credentials (med school, US training, USMLE, ECFMG).
FCVS establishes a permanent, lifetime repository of primary-source verified core credentials for physicians that is accepted by all medical boards except AR. CA will accept an FCVS profile on a case-by-case basis, so you might still have to get your medical school to complete CA forms L2, L5, L6, and provide transcripts and a certified copy of your medical diploma.
There are a few boards that require an FCVS profile: Kentucky, Louisiana, Maine (also requires verification of postgraduate training performed in England, Ireland and Scotland), Nevada (Osteo), New Hampshire, New York (IMGs only), North Carolina (IMGs or those with a profile already established), Ohio, Rhode Island, South Carolina, Utah (Medical and Osteo), the Virgin Islands and Wyoming.