DENVER (AP) — A federal judge has blocked Colorado from enforcing a new law raising the age to purchase a gun from 18 to 21.

U.S. District Judge Philip Brimmer issued a preliminary injunction Monday following a legal challenge by a gun rights group. In a ruling that frequently referenced a U.S. Supreme Court ruling that expanded Second Amendment rights last year, he concluded that the lawsuit brought by Rocky Mountain Gun Owners would likely succeed and barred the state from enforcing the law until the case is resolved.

The law was one of four gun control bills signed by Democratic Gov. Jared Polis in April, following the lead of other states trying to confront a surge in violent crime and mass shootings.

Last year’s Supreme Court decision in a New York case changed a test lower courts had used for evaluating challenges to gun laws, threatening to upend firearms restrictions across the country.

Colorado’s law effectively sought to prevent those between 18 and 20 from buying rifles and shotguns. A federal law already prevents licensed firearms dealers from selling handguns to those under 21 but that ban has also been challenged in light of the Supreme Court decision.

A spokesperson for Polis, Conor Cahill, called the difference in age limits for different types of guns a “loophole.”

“The Governor is working towards his goal of making Colorado one of the ten safest states in the country — and the same age requirements for pistols and rifles would help support responsible gun ownership,” he said.

In a statement, Rocky Mountain Gun Owners’ executive director, Taylor Rhodes, said the group warned Colorado lawmakers the effort would be struck down in court.

“Today, our crystal ball became a reality. But it doesn’t stop here. We won’t stop fighting until every single unconstitutional anti-gun law is struck down,” Rhodes said.