Members of the United Constitutional Patriots share cigarettes while patrolling the U.S.-Mexico border in Sunland Park, N.M., in March. The American Civil Liberties Union opposed the militia's actions in an April letter that asks New Mexico's governor and attorney general to investigate the group. (Paul Ratje /AFP/Getty Images)
Shahrukh Khan is a J.D. candidate at Emory University School of Law.

The United Constitutional Patriots, an armed militia at the southern border in New Mexico, has drawn intense scrutiny after live-streaming its activities on social media. Groups like the UCP routinely litter parts of the border with traps of all brands to discourage — and now detain at gunpoint — migrants from coming to U.S. territory. They patrol the Mojave in the dead of night and coordinate with Customs and Border Patrol agents to transfer the people they have kidnapped into government hands.

Yet while these actions have been met with significant outrage (and ultimately the arrest of UCP leader Larry Mitchell Hopkins on federal firearms charges), the group argues that it is acting lawfully, in the tradition of other vigilante groups operating along the U.S.-Mexico border. For all the noise they make, these groups have come to use one practice that, at first blush, is quite meek, and almost arcane in today’s world: the citizen’s arrest. But thanks to critical aspects of U.S. law, this practice is rife for abuse and gives these groups far more leeway to act than one might think.

Originally standing in place of official police departments in Europe, the citizen’s arrest was a practice deployed by royalty to use ordinary people to sniff out criminals. The principle was first created under common, or judge-made, law in the Statute of Winchester in 1285, enacted by King Edward I to replace the existing watch and ward system. It outlined the role that private citizens played in criminal justice and provided that anyone who witnessed a crime would “make hue and cry” until the criminal was caught.

As the doctrine developed through common law, a sharp distinction on the basis of discretion arose between private citizens and peace officers: A private citizen could only arrest another for a crime committed in his or her presence, whereas police officers could arrest based on third-party accounts. Because of the vagueness regarding discretion, many U.S. states have chosen to codify and clarify this right. But these laws are not uniform, leading to a patchwork of different formulations in different states.

In California, for example, the presence requirement has been broadly interpreted to the point where physical proximity, sight or intent is not required to make an arrest. In Padilla v. Meese, a citizen asked a suspect he did not know to pull over and wait for the police to arrive. This would not have been legal in other states.

The vagueness and inconsistency about just what citizens could do in terms of arresting their fellow citizens have persisted for several reasons. The rise of modern police departments depreciated the importance of training private citizens in effecting a safe, lawful arrest. Additionally, uneven experiences have helped to shape the law. Individual cases, especially controversial or newsworthy ones, have prompted courts to create new standards or assess the constitutionality of the citizen’s arrest. Over time, these have created a corpus of law in each place.

This diverse landscape and a lack of public education about how to safely carry out a citizen’s arrest have led legal scholar Ira Robbins to argue that the practice is “ripe for abuse.” And for many migrants and Americans who have attempted to help them, abuse has apparently been rampant in recent years.

New Mexico’s government criticized the UCP after a video surfaced in which armed men were detaining migrants who had just crossed the border. But the state is limited in terms of how much it can crack down on the vigilantes thanks to a core provision of the Constitution. Thanks to the Supremacy Clause, if a state law conflicts with a federal law, then the federal law “preempts” or invalidates the state law. A particular kind of preemption called “field preemption” is triggered when a scheme of federal law and regulation (such as immigration, foreign policy or war) is “so pervasive as to make reasonable the inference that Congress left no room for the states to supplement it.”

This means that states can only adopt their own policies on matters touching on immigration under narrow circumstances. In a case pitting Arizona against the United States regarding a state law dealing with immigrants and checking immigration status, Justice Anthony M. Kennedy, writing for a Supreme Court majority, made clear that the “federal government has occupied the field of alien registration.” Most of the act’s provisions were struck down, and the ones that were upheld were interpreted very narrowly.

So while, theoretically, local and state governments have influence over how the practice of citizen’s arrests can be used in their jurisdiction, this is not the case with regard to immigration. State governments are almost powerless to curtail the vigilante, extrajudicial operations of militia groups working in conjunction with federal authorities.

For that reason, those sympathetic to more open immigration might need to think seriously about how state governments can work through and around the Supremacy Clause to have a greater stake in the issue. States such as California have enacted sanctuary policies, which prohibit state law enforcement from contracting with federal authorities to house people as federal detainees for civil immigration custody. Such creative policies may be necessary if states want to stop vigilante groups focused on immigration from operating in their states. That being said, those sympathetic to open immigration should be careful about how much faith they place in states to solve immigration problems: Tennessee has barred state and local officials from adopting sanctuary policies.

Ultimately, because of Supreme Court precedent and the Supremacy Clause, it is very likely that a solution to the migrant crisis will give way to the triumph of federal policy. Indeed, the oft-forgotten practice of citizen’s arrest reminds us that the messiness of our system, which dates back to the delicate compromises that enabled the United States’ foundation, have left us with a dizzying patchwork of law that can often be hard to harness and direct.