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PHOTO: Scott Olson/Getty Images North America/Getty Images
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(CNN) —  

The Trump administration separates immigrant families only when it’s necessary to protect the children, the secretary of homeland security said Wednesday, though she added that in at least one case the process “took too long.”

Advocacy groups have been accusing the Department of Homeland Security of separating immigrant families in detention or as they await court proceedings, though there has been no clear evidence it is a widespread practice.

Homeland Security Secretary Kirstjen Nielsen said Wednesday at a hearing of the House Homeland Security Appropriations Subcommittee that there is no policy that encourages the separation of parents from their children as a punitive or deterrence measure – and it happens only when there is doubt about whether an adult may really be a parent or legal guardian of the child they’re with or when the child might be in danger.

“The standard is to – in every case – is to keep that family together as long as operationally possible,” Nielsen said. “When we separate, we separate because the law tells us to, and that is in the interest of the child. … Unfortunately, we have seen instances where traffickers have used children to cross the border and gain access illegally.”

But Nielsen did say that in one high-profile case that generated a lawsuit, the process of verifying a familial relationship took longer than it should.

“In the Congolese case that you mentioned, it took too long,” Nielsen told Rep. Lucille Roybal-Allard of California, the top Democrat on the subcommittee, who brought up the case. Nielsen added that due to ongoing litigation she couldn’t say more, “but it took too long, so we are looking at that particular case.”

In that case, the American Civil Liberties Union sued on behalf of a Congolese woman who said she had been separated from her 7-year-old daughter for months. They were eventually reunited, and a DNA test confirmed the woman was the child’s mother, according to court filings. The lawsuit, which is seeking class-action status, continues.

Nielsen also said the department is working with embassies and consulates overseas to help immigrants understand what protections they can apply for and to have the proper paperwork to verify they are families in advance.