A young child who may be American, but is among the thousands classified as unaccompanied minors after being separated from parents near the Mexican border, has been in federal custody since 2015, the government acknowledged in a Thursday court filing.

The child, who is under the age of five, had been with a parent who was apprehended at the border on an outstanding warrant by the U.S. Marshals Service. In Thurday's court filing, lawyers for the government wrote that the parent "may or may not be a United States citizen." On Tuesday, the lawyers wrote "records show the parent and child might be U.S. citizens."

A representative for U.S. Health and Human Services did not reply to questions from CBS News about the type of facilities where the child has been housed since 2015. HHS oversees a foster care network as well as more than 100 childrens' shelters.

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The Justice Department's court filing twice refers to the child as "her," but a representative of U.S. Immigration and Customs Enforcement said that agency's records indicate the child is a boy. CBS News has reached out to multiple federal agencies for clarification.

"The adult described in the court filing presented herself and her minor son, born in Mexico, at a U.S. port of entry on the Texas border on Nov. 25, 2015," said ICE spokesperson Danielle Bennett in an email to CBS News. "The parent was remanded to USMS custody on an outstanding warrant. At no time did she enter ICE custody."

Although when separated the parent was in the custody of the U.S. Marshals, a federal law enforcement agency, the government has since lost track of the parent, according to the filing.

"Defendants (the government) have not been aware of the parent's location since then and they remain unable to locate that parent," Justice Department lawyers wrote in the court filing.

It is not clear if the parent was charged, jailed, released, or transferred to any other federal agencies after the 2015 separation.

"Because the parent is not available, it is not possible to reunite the child with the parent. Unless the parent is located, [HHS] will provide care and seek placement for the child using its ordinary programs and procedures," the Justice Department lawyers wrote.

Lee Gelernt, an ACLU lawyer involved in the lawsuit demanding that separated children be reunited, said in a text message to CBS News that the organization is attempting to track the mother down.

"The government doesn't know where the mother is now, so we will try to find her," Gelernt said, adding that the ACLU has not been told the woman's identity.

In response to questions about the mother, a spokesperson for the U.S. Marshals referred CBS News to the Justice Department, which declined to comment.

The child is one of 103 children under the age of five who were separated from their parents near the border, before a federal judge ordered the government to reunite them by July 10. The government has since ruled 46 of those children, including the possibly-American child, are currently ineligible for reunification.

Reasons for ineligibility range from the parent having already been deported to the adult not actually being a parent — though in some cases those ruled ineligible are relatives such as grandmas and an uncle —and failures of criminal background checks. Criminal histories referenced range from the violent and sexual to some not always associated with the loss of childcare privileges, such as three adults who had previous drunk driving arrests.

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