I am almost positive that clearing customs would be at Dulles. Upon returning to the U.S. and landing in, say, Charlotte or Miami, I clear through there and then get my connecting flight (after rechecking my bags once again) back to Boston. If a direct, non stop flight to Boston, I clear in Boston.
It can be a bit of a mini ordeal.
Luanne, maybe if you call the airline they can give you a definitive answer.
Addendum: ETA:
Found it:
Customs at Dulles
Welcome to the United States
Passengers arriving at Dulles on International flights will be directed to U.S. Customs.
Note: Passengers arriving from the Bahamas, Aruba, Bermuda or Ireland may have already pre-cleared U.S. Customs at the point of departure. Through this system, travelers are treated as domestic passengers upon arrival and will deplane directly into the Concourse. From there passengers can go directly to their connecting flight or to the Main Terminal and exit the airport.
At U.S. Customs there are two queue lines: one for U.S. Citizens/Permanent Residents and one for Non U.S. Citizens. Please ensure you are in the correct line in order to speed the process. Wait times vary depending on the number of arriving international flights at that time. Please have completed customs forms handy when you reach the front of the line.
After clearing U.S. Customs at Washington Dulles International Airport, passengers must claim their checked luggage before proceeding in to the Main Terminal or to a connecting flight. All luggage in the U.S. Customs area is subject to inspection. To check-in luggage for a connecting flight, simply hand it to an agent in the baggage re-check area before exiting.
Passengers transfering to a connecting flight will need to clear screening by the U.S. Transportation Security Administration (TSA). After entering the Main Terminal, go up one flight to the Departures (Ticketing) level. Present the boarding pass for your next flight along with photo identifcation. Click here for more detailed information about TSA screeening guidelines.
For more information:
Please click here to visit the U.S. Customs and Border Protection website.
Global Entry (International Registered Traveler) program for U.S. Citizens - click for more information.
Click here for the Electronic System for Travel Authorization (ESTA) website.
The phone number to U.S. Customs at Dulles is 703-661-2800.
U.S. Residents, click here for information about obtaining a Passport.