The seal of the Department of Homeland Security. (Mandel Ngan/AFP/Getty Images)

WHO KNOWS why Homeland Security agents in Southern California forcibly separated a 7-year-old Congolese girl from her mother last fall, flew her 2,000 miles to Chicago, where she was placed at a facility for unaccompanied minors, and kept her there for more than four months? Who knows why the girl, who is credibly reported to have been traumatized, has been permitted to speak with her mother, only recently released from a detention center near San Diego, just a handful of times in the intervening four months? And in the absence of any evidence of wrongdoing by the mother, who presented herself to U.S. officials when she crossed the border from Mexico, who knows why the government has continued to keep parent and child apart?

The Department of Homeland Security has declined to comment on the case of the two asylum seekers, known in court filings as Ms. L and S.S. But a spokesman said in a statement that agents may separate children and adults if they suspect the child may be a human-trafficking victim. “If we are unable to confirm this relationship [between adult and child],” said the spokesman, Tyler Houlton, “we must take steps to protect the child,” including placing her in a facility for unaccompanied children.

In this case, DHS’s effort to establish Ms. L’s guilt by insinuation failed, and its stated concern for the child’s protection and well-being has been exposed as phony. For four months, no testing was performed to establish the woman’s maternity. And when, following a lawsuit filed on their behalf, the two were finally subjected to DNA testing this month, the result was unequivocal: Ms. L is the mother of S.S.

That finding has been met with silence by DHS. The department, having originally expressed indignation at the idea that it would separate children from their parents for any reason other than the child’s welfare, has been rendered speechless.

U.S. officials who interviewed Ms. L when she crossed the border made a preliminary finding that she had a plausible claim for asylum, based on her account of having fled what the lawsuit, filed by the American Civil Liberties Union, said was “near certain death” in Congo. Despite that, she was detained until the lawsuit and ensuing publicity prompted her sudden release last week.

In a class-action suit, the ACLU asserts that the Trump administration has separated children from their parents in more than 100 cases, even though the department says it does not “currently” have a policy on the matter. If it seems unthinkable that the administration and Homeland Security Secretary Kirstjen Nielsen would carry out a practice so cruel, one likely to inflict long-term harm on children, think again: DHS officials, including Ms. Nielsen’s predecessor, John F. Kelly, now the White House chief of staff, have said they believe it would be an effective means of deterring asylum seekers.

If DHS has subjected this small girl to trauma as a warning to other asylum seekers, it is an unconscionable means to an end. If that is not the reason, then what is?