Any traveler arriving into the United States is processed by the U.S. Customs and Border Protection, even if they are taking a connecting flight that is leaving the United States. You must have a Visa or be eligible for the Visa Waiver Program to transit the USA. Once you pass through immigration you must collect your baggage and proceed through Customs. After you clear U.S. Customs, there will often be airline counters available and you will be required to check-in for your connecting flight. If you arrived on a Visa or the Visa Waiver Program, you must return the I94 or I94W slip to the airline so your departure from the United States can be recorded. When you head to your gate for your connecting flight, you will have to be screened by the Transportation Security Administration since you left the secure area of the airport. To be on the safe side, you should ensure that you have at least 3 hours to transit the United States before your connecting flight leaves.
For a list of documents you require to enter the United States,click here.