A 9-year-old United States citizen was detained at a Southern California border checkpoint for more than 30 hours this week while the authorities said they worked to verify her identity.

The girl, identified by NBC 7 San Diego as Julia Isabel Amparo Medina, had been making her daily commute from Tijuana, Mexico — where she and her 14-year-old brother, Oscar, live — to school in San Ysidro, Calif., on Monday, the news station said. Because traffic was moving slowly, Julia and Oscar opted to walk across the border rather than wait in the car and risk tardiness, according to NBC 7, which first reported the story.

In an email on Friday, a Customs and Border Protection spokesman said two children arrived at one of the San Ysidro port of entry facilities for pedestrians at 10:15 a.m. on Monday. The children, whom the spokesman did not identify, presented a C.B.P. officer with United States passport cards, according to the spokesman, Ralph DeSio.

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