Federal law enforcement officials are using a long-dormant law to detain immigrants and charge them with willfully failing to register with the government.

Under the Alien Registration Act, passed in 1940 because of fears about immigrants' loyalty to the United States, almost all noncitizens must register with the government and submit fingerprints, reports Politico on Saturday.

The law has remained on the books since that time, but has remained dormant until President Donald Trump issued orders to enforce the law. As a result, since April, people have been arrested in Louisiana; Arizona; Montana; Alabama; Texas; and Washington, D.C., and charged with willful "failure to register" under the federal registration law.

Most of those charged were already in jail and in proceedings for deportation when the new charges were presented against them.

Failure to register is considered a misdemeanor and brings maximum penalties of six months in jail or a $1,000 fine.

The Trump administration's push to enforce the law means that immigrants who register must submit detailed and potentially incriminating information about themselves to the government. However, refusing to register is a crime that is punishable by arrest or prosecution, adding to the threat of being deported.

The Trump administration also says a purpose of reviving the dormant rule is to push undocumented immigrants to choose to leave the United States willingly.

"For decades, this law has been ignored — not anymore," the Department of Homeland Security said in an announcement in February about enforcing the law.

The department says "mass self-deportation" is a "safer path" to remove immigrants and claims that when people leave on their own, it will save taxpayers' money.

The Alien Registration Act also includes a separate provision that criminalizes plots to overthrow the government. The provision was used for about 20 years while prosecuting people it accused of being pro-Communist or pro-fascist.

The registration provision, however, had not been enforced for 75 years. It applies to noncitizens who are in the country for 30 days or longer.

Some immigrants have met the requirement through filing applications to become permanent residents, or have received "parole" from deportation.

However, the DHS says that about 3.2 million immigrants are currently unregistered, making them subject to facing charges.

The administration has also created a new seven-page form for noncitizens to fill out. And under the penalty of perjury, they must provide biographic details, contact information, and more.

The enforcement actions are being contested in court, with the Coalition for Humane Immigrant Rights and other advocacy groups filing a lawsuit in March.

U.S. District Judge Trevor McFadden, a Trump appointee, at first was skeptical about the use of the legislation. But in April, he refused the coalition's request to block the policy, saying that it did not show that it would be harmed.

The prosecutions under the law so far have stumbled, including in May, when a federal magistrate judge in Louisiana consolidated and dismissed five cases and ruled the prosecutors had no probable cause to show the defendants willfully refused to register.

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