Begin with identity and family records, prior U.S. immigration documents, and evidence explaining the lawful origin of the proposed funds. Preserve the records linking those funds through accounts and transactions. The precise supporting documents depend on the funding history and current filing requirements; a single account statement rarely explains both origin and movement.
Build a funding chronology that survives a change of project
For each funding component, record the event that created the money, the person or entity entitled to it, and the accounts through which it moved. A business sale, inherited asset and employment savings will have different supporting records. Obtain prescribed historical tax documents and other records required for the actual case, rather than assuming a universal short list.
Keep originals and identify translation needs before bank archives or former advisers become difficult to reach. Build the chronology as a table with one row per event and one column for the document that proves it, then mark the rows where no document yet exists. Older events are the ones that fail, since institutions purge records and the people who could explain a transaction retire or become unreachable.
Hypothetical example: proceeds from selling a share in a small audiobook production company twelve years ago may be provable only through the buyer's own accountant, and that route closes with time. Work on the oldest unproven row before the largest one, and keep a log of every request made so an unanswered one can be chased.