Ngtjyty

Let's say I need to bring over $10,000 into the US and decide to use cash to save on transfer fees. What would happen at the border when I declare all that cash? Would CBP briefly question me on the source of funds or require some proof to confirm that the funds are from a legal source?

Are there any official guidelines on this matter?

I've done this once (doing all the right paperwork up front). They pulled us out for questioning: "where does the money come from" "why do you need this" "what are you planning do to with this", "why do you use cash", etc.

The amount of questioning will depend on how much you fit into any "suspicious" category otherwise. There is plenty of profiling going on there. Fortunately we were "mostly harmless" for everything else (white, middle class, small children it tow, from a "good" country).

You also need to be careful how this looks against your visa status. If you have a non-immigrant Visa or an ESTA, carrying lots of cash can be interpreted as "intent to immigrate", in which case they can turn you around and sent you back.

Unfortunately customs and immigration officers in the US have a lot of leeway and discretion and not a lot of rules to keep them grounded. Could be harmless, could be lots of trouble, hard to predict.

You simply fill out a form at the border. See the official CBP page Currency / Monetary Instruments - Duty on money, checks, etc.

Travelers leaving or entering the U.S. are required to report
negotiable monetary instruments (i.e. currency or endorsed checks)
valued more than $10,000 on a "Report of International Transportation
of Currency or Monetary Instruments" FinCEN Form 105. The requirement
to report currency on a FinCEN 105 does not apply to imports of gold
bullion. You can obtain the form at www.fincen.gov or request one from
the CBP Officer if required.

Failure to declare currency in amounts more than $10,000 can result in
its seizure.

From Form 105

Travelers carrying currency or other monetary instruments with them
shall file FinCEN Form 105 at the time of entry into the United States
or at the time of departure from the United States with the Customs
officer in charge at any Customs port of entry or departure.

and

PENALTIES: Civil and criminal penalties, including under certain
circumstances a fine of not more than $500,000 and Imprisonment of not
more than ten years, are provided for failure to file a report, filing
a report containing a material omission or misstatement, or filing a
false or fraudulent report. In addition, the currency or monetary
instrument may be subject to seizure and forfeiture. See 31 U.S.C.5321
and 31 CFR 1010.820; 31 U.S.C. 5322 and 31 CFR 1010.840; 31 U.S.C.
5317 and 31 CFR 1010.830, and U.S.C. 5332.

NOTE that the form itself contains no fields requiring you to justify where the funds were obtained from. However I would expect that declaring the money would trigger some sort of process that flags you as having carried the money across the border. I also expect that whether or not you are questioned more throughly would depend on a lot of things from who you are to where you are coming from.

NOTE also that simply carrying a large sum of currency in the US puts you at risk of Civil Forfeiture by local police departments. There have been many cases of people carrying legitimately obtained funds being stopped by police (for example traffic stop) and having those funds confiscated. You do not even have to be convicted of a crime for this to happen. As such I don't recommend traveling with large sums of cash in the US.