The incident happened at about 10:30 a.m. at the domestic baggage claim area. The girl's injuries were described as lacerations, with one bite to abdomen, a spokesperson said.
The girl was taken to an area hospital immediately for treatment. Her condition is unknown at this time.
"We do not know what exactly happened to cause the dog to bite the girl," CBP spokeswoman Kelly Ivahnenko said. "We're still trying to get to the bottom of what happened."
As far as other CBP dogs biting people, Ivahnenko said she had "no idea" when or if the situation had occured before, but stressed that "this is not a common occurence."
She also said that the safety of travelers entering and exiting the U.S. is a paramount concern for the agency. "That's a primary focus for us," Ivahnenko said.
The dog, a Belgian malinois, was in training at the CBP's training facility in Front Royal. Dogs from that facility are frequently brought to Dulles for real world training and experience. In this situation, the handler had brought the dog to the domestic baggage area for training.
The dog has been returned to the Front Royal training facility.