By Clare Foran and Tal Kopan, CNN

(CNN) -- Four members of the Homeland Security Advisory Council, which advises the Department of Homeland Security, have resigned, saying in a letter that they can no longer be associated with the Trump administration's immigration policies and calling the separation of migrant families "morally repugnant."

The letter, which was obtained by CNN, was addressed to Homeland Security Secretary Kirstjen Nielsen and signed by Richard Danzig, David Martin and Matthew Olsen -- all former Democratic administration officials -- and by Elizabeth Holtzman, a former Democratic congresswoman from New York.

The four said in the letter that "unfortunately," they had not been consulted "before recent immigration decisions were announced, enforced, and then retracted by this administration" and that had they been consulted, they would have "observed" that family separations were not only morally repugnant, but also "counter-productive and ill-considered."

Last month, President Donald Trump abruptly reversed course after facing a backlash over the separation of undocumented families at the border and signed an executive order to halt it. The US government has since been ordered by a federal judge to reunite separated families, a process that is ongoing. CNN has previously reported that the government hasn't been able to locate the parents of 71 children who were likely separated from them.

The resignations were first reported by The Washington Post.

Homeland Security spokesman Tyler Houlton called the resignations "disappointing, but not surprising."

Houlton said, "It's disappointing, but not surprising, that appointees from the previous administration would resign their positions on the Homeland Security Advisory Council over enforcement of immigration law -- as written and passed by Congress."

He added, "The practice of prosecuting individuals for breaking the law has been carried out by previous Administrations, including President Obama. It is unfortunate that instead of first bringing their concerns directly to the Secretary in the spirit of an Advisory Committee member, they chose to simply resign four weeks after the Administration ended the practice of concern. DHS has many missions and protects the homeland against a multitude of threats. The HSAC provides valuable advice to the Secretary and the Department looks forward to engaging new members to be announced soon."

The council, which is permitted to have up to 40 members, currently has around two dozen, according to a DHS website.

In a separate letter to Nielsen, Martin further criticized the administration's separation of families, saying it had been "executed with astounding casualness about precise tracking of family relationships -- as though eventual reunification was deemed unlikely or at least unimportant, even for toddlers and preschoolers." He added that the separations had "crystallized" for many advisory council members "profound doubts about the administration's commitment to the rule of law."

Holtzman also elaborated on her objections in a separate letter to Nielsen, writing, "Under your administration and that of Donald Trump's, DHS has been transformed into an agency that is making war on immigrants and refugees." Holtzman went on to write that the separation of children from their parents amounts to "child kidnapping, plain and simple."

"Seizing children from their parents in violation of the constitutional rights of both is bad enough (mentally harmful to the children and infinitely painful to both the parents and children), but doing so without creating proper records to enable family reunification shows utter depravity on the part of the government officials involved," Holtzman wrote.

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