Updated at 1:33 p.m. ET

A federal judge in Michigan has dropped most of the charges against a Detroit doctor accused of female genital mutilation, concluding that Congress "overstepped its bounds" when it passed a law banning the practice.

That 1996 law violates the Constitution and is unenforceable, the judge concluded, because in general, criminal law is left to the states — and female genital mutilation should be no exception.

As a result, he dropped six federal charges against Jumana Nagarwala, who was accused of mutilating the genitals of multiple girls when they were about 7. Other defendants charged with assisting in the procedures have also had charges dropped.

Two charges against Nagarwala, for "conspiracy to travel with intent to engage in illicit sexual conduct" and obstruction, remain pending.

A spokeswoman for the U.S. Attorney's Office for the Eastern District of Michigan told NPR that the office is "reviewing the judge's opinion and will make a determination whether or not to appeal at some point in the future."

A lawyer for Nagarwala tells NPR that the ruling is "exactly what our justice system is designed to do."

"The judge found Congress, through the United States Constitution, lacked authority to enact the FGM statute," attorney Molly Blythe told NPR via email. "The law warranted this decision, and we are happy with it."

An attorney for Fakhruddin Attar — a doctor accused of allowing Nagarwala to use his clinic to perform the procedure, charges that were dropped in this decision — told NPR via email that the decision "is truly a victory for everyone because the court required the government to adhere to the mandates of our Constitution."

"We're thrilled with the court's well-reasoned and thoughtful opinion," Mary Chartier said. "We are confident that a trial would have shown that the religious procedure done in these cases was protected by the Constitution and far less invasive than male circumcision. No matter what future court proceedings may still occur, we're committed to fighting for as long as it takes to prove Dr. Attar is innocent."

This case, filed in early 2017, is believed to be the first ever brought under the federal law against female genital mutilation, according to the U.S. attorney's office.

Lawyers for the U.S. government say Nagarwala performed female genital mutilation on at least nine girls.

In court filings, Nagarwala denies that she conducted female genital mutilation, saying that the procedure she performed "did not involve the clitoris or labia," as one filing put it.

As NPR's Merrit Kennedy reported last year, one young victim "told federal agents that she underwent a procedure 'to get the germs out' of her, and she identified Nagarwala in a photograph as the doctor who performed the operation. A medical examination performed under a search warrant found that her 'labia minora has been altered or removed, and her clitoral hood is also abnormal in appearance.' "

Another victim, according to court documents, said that she received a painful "shot" from Nagarwala that caused her to scream and left her barely able to walk and that her parents told her not to talk about the procedure.

Kennedy reported:

"There are multiple types of FGM, including altering or removing the clitoris, labia minora and/or majora. Rights groups condemn it as an attempt to control the sexuality of women — which is also how many of its proponents justify it. ...

"FGM is practiced in dozens of countries, most commonly in Africa, but also in parts of the Middle East, Eastern Europe and South America. It is not restricted to members of a single faith; according to the U.N.'s Population Fund, it is practiced by some Muslim groups, 'some Christians, Ethiopian Jews, and followers of certain traditional African religions.' That's why the U.N. views it as a cultural practice, rather than a religious one."

The practice was outlawed in the U.S. more than two decades ago. But Bernard Friedman dismissed all charges against Nagarwala stemming from that law. He noted that criminal law in the United States is generally defined and enforced by states — not by the federal government.

There are exceptions, he said, but only where the federal government has specific authority to pass laws.

For instance, Congress can regulate interstate commerce — but female genital mutilation does not appear to be a "commercial activity," the judge said, noting there was no evidence of money changing hands.

And health care can be regulated by the federal government, but the judge said female genital mutilation is "a form of physical assault," not a health care service.

U.S. attorneys also argued that Congress has the right to pass a ban on female genital mutilation because of a treaty promising equal protections for men and women, and promising to protect children from discrimination based on sex, race or other characteristics.

But because the ban on female genital mutilation is specific to girls, the judge did not see the relevance of that treaty. "As laudable as the prohibition of a particular type of abuse of girls may be, it does not logically further the goal of protecting children on a nondiscriminatory basis," he wrote.

In short, female genital mutilation is a crime — an assault against a child — which should be prosecuted by the states, not the federal government, he ruled.

"I'm angry that the federal judge dismissed this horrific case that affected upwards of a hundred girls who were brutally victimized and attacked against their will," Michigan state Sen. Rick Jones said in a statement. "This is why it was so important for Michigan to act. We set a precedent that female genital mutilation will not be tolerated here, and we did so by passing a state law that comes with a 15-year felony punishment. I hope other states will follow suit."

Jones was one of the co-sponsors of Michigan's legislation banning the practice.

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