Former Secretary of Homeland Security Kirstjen Nielsen was interviewed this week as part of FORTUNE’s “Most Powerful Women Summit” in Washington. Nielsen, who seemed nonplussed that her interviewer, PBS NewsHour’s Amna Nawaz, intended to conduct a serious interview and not simply sit back and let the former Trump administration official recite her cybersecurity talking points, doubled down on her role in family separation, stating that she did not regret “enforcing the law.”

While prior presidential administrations have certainly struggled with periodic migration and border security challenges, officials in the Trump administration revived a controversial proposal that had been considered briefly during the Obama administration, but quickly shelved as “too opprobrious and unpalatable.” Once implemented last year, the early effects of the family separation policy and practice were swift, and devastating: Children were separated from their parents or family members at the border as a consequence of a new prosecutive guideline from the Justice Department. Parents were given little or no information about where the children were re-located to, or when, if ever, the families would be reunited. While statistics available as of early fall 2019 indicate that over 4,000 children were separated from their parents at the southern border, the numbers continue to go up, including new information from the ACLU that more “tender age” children were separated than previously disclosed. It took advocacy litigation and a court order to begin the process of reunification, a process that continues even today.

As far as we know, until 2018, the U.S. government had not previously implemented a policy and practice of intentionally separating migrant and asylum-seeking families as a means of deterrence. As Americans, each of us was horrified our government would rip vulnerable children from their families in such a deliberate way. Each of us observed the developments from different political and personal perspectives, but collectively, as lawyers and law professors, we wondered how the government’s activities were even arguably legal under either domestic or international law. The Trump administration’s family separation policy shocked our respective consciences.

So, we set to work. As one component of the Center for a New American Security’s project on DHS Oversight & Accountability, we spent the summer of 2019 surveying law relevant to this administration’s implementation of family separation. Drawing from our combined research and experience in constitutional, domestic and international law, we set out to explore and explain the numerous areas of law implicated. We concluded that diverse legal grounds all indicate the unlawfulness of family separation as a policy tool for deterring immigration. A pre-publication draft of the resulting article, The Law Against Family Separation, is now available on SSRN, and will be published in final form by the Columbia Human Rights Law Review, in early 2020. Here we provide a brief précis.

Our article comprehensively surveys and assesses relevant law to (i) highlight the unlawfulness of the policy that the Trump administration implemented, (ii) identify areas where Congress can step in to prevent a future administration from implementing something similar, and possibly aid in remedying the effects of the Trump policy on separated families, and (iii) provide international context that takes family separation off the table entirely as a policy option, even in the face of significant migratory challenges.

We begin by situating family separation in the context of immigration and border security, and the federal government’s powers in these areas. While the federal government has well-established constitutional and statutory authority to enforce immigration law and ensure border security, the exercise of these powers is not limitless. Enforcement activities must conform with constitutional requirements of due process. They are lawful exercises of national security-related power only if national security justifications are legitimate, not pretextual; and even in the face of actual national security concerns, the government must respect the right to family integrity and basic constitutional protections

Next, we move from broad discussion of government power and constitutional constraint to examine the law relevant to separated families seeking individual redress for the harms inflicted on them by the U.S. government. They may be able to seek damages from federal officials in federal civil rights actions, in claims of deprivation of their constitutionally protected right to family integrity. People subjected to family separation under Trump’s policy could also likely bring claims in tort, particularly actions for intentional infliction of emotional distress. Finally, we consider the applicability of international law, emphasizing that special protections afforded to children, requirements for humane treatment of migrants, and principles of family integrity are all relevant in defining the limits of what is an acceptable, and lawful, government practice.

Comprehensively assessing the law against family separation is a complex undertaking. The underlying practice—deliberate, harsh, and large-scale family separation as a deterrent to migration—is unparalleled in U.S. history, so it is not necessarily obvious which bodies of law apply, and how. As it turns out, many types of law prohibit this practice, though some could use bolstering by congressional action. Our article, therefore, is a broad survey of applicable areas of law with concrete discussion of how to use it. We look at an expansive range of currently applicable law, consider the viability of different specific legal approaches, and make an initial effort to identify the kind of federal legislation that could address the Trump administration’s family separation policies and practice beyond the important and consequential litigation that has already been brought. Lawyers, lawmakers, policy experts and citizens must work together to ensure that legitimate immigration law and border security enforcement objectives are achieved without resorting to policies and practices that lack sound legal grounding and take place at severe costs to the most vulnerable among us.

Image: Honduran father Juan and his six-year-old son Anthony walk on their way to attend Sunday Mass on September 9, 2018 in Oakland, California. They fled their country and crossed the U.S. border at a lawful port of entry in Brownsville, Texas seeking asylum. They were soon separated and spent the next 85 days apart in detention. Juan was sent to Tulsa, Oklahoma, while his son was sent to a detention shelter New York. Juan said it took six weeks from the time of separation until he was able to make a phone call to his son. Photo by Mario Tama/Getty Images