As the federal government races to reunite the children it separated from parents, one child under the age of 5 appears to stand alone: He or she may be an American citizen.

In a court filing Tuesday, the Justice Department included the child in a section called: "Not eligible for reunification." Its explanation: "1 child cannot be reunified at this time because the parent's location has been unknown for more than a year. Defendants are unable to conclusively determine whether the parent is a class member, and records show the parent and child might be U.S. citizens."

A spokesperson for U.S. Immigration and Customs Enforcement said in an email to CBS News that the child's parent is not currently in ICE custody. The child is currently in the custody of the Department of Health and Human Services' Office of Refugee Resettlement. A spokesperson for the agency said it could not provide more details about what indication there is that this child and parent may be U.S. citizens, how the child came to be in federal custody or where the child is currently sheltered.

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Lee Gelernt, an ACLU lawyer involved in the lawsuit that led a federal judge to order the government to reunite families that were separated after crossing the Mexican border, said in text messages to CBS News that the ACLU has been unable to learn anything more about the child.

Gelernt added that the child never should have landed in the custody of immigration officials.

"Unfortunately we don't have additional info yet," Gelernt said. "We are trying to find out ASAP, because that child of course should not have been in immigration custody in the first place."

As of midday on Tuesday, the government had reunited four children under the age of five with their parents, but said in court documents it might be able to reunite as many as 51 more that day. The government has not since publicly updated its figures, but it is scheduled to submit a new filing to the court Thursday.

The government has until July 26 to reunite nearly 2,800 other children, ages five through 17, currently in its custody.