Connecticut’s Supreme Court ruled Thursday that the manufacturer of the Bushmaster AR-15 semi-automatic rifle can be sued and potentially held liable in the 2012 mass murder at Sandy Hook Elementary School in Newtown, Connecticut.

The groundbreaking 4-to-3 decision overcame a federal law backed by the National Rifle Association and designed to immunize gunmakers from liability for the crimes committed with their weapons. Remington makes the Bushmaster.

The suit was brought by the estates of nine victims of Adam Lanza, who was armed with the high-powered rifle during his assault that left 28 dead, including 20 young children.

The court’s decision was narrow, with the liability for gunmakers based on how they advertise their weapons rather than on the sale of them to third parties who then commit horrible crimes. In its ruling, the court said companies that market military-style guns to civilians as a way of killing enemies could be violating state fair trade laws.

“The regulation of advertising that threatens the public’s health, safety, and morals has long been considered a core exercise of the states’ police powers,” the court said, explaining why the federal law does not apply.

Neither Remington nor its lawyer in the case could be reached for comment Thursday. A spokesman for the company told the Associated Press that, “We have no timeline for any comments to be made on the subject.”

The families alleged, for example, that the Bushmaster product catalogue shows soldiers on patrol in jungles along with the phrase: “When you need to perform under pressure, Bushmaster delivers,” and promotes the rifle to civilians as “the ultimate combat weapons system.”

Still, the decision was a breakthrough for those affected by the Sandy Hook shooting and for gun-control advocates across the country who have found little to no success in seeking ways to hold gunmakers responsible for crimes committed with their products.

“This is a landmark and potentially historic ruling,” said Adam Winkler, a UCLA Law School expert in gun regulation. “It opens up an avenue to hold gunmakers responsible despite federal immunity. It will encourage a lot more litigation” against them as well as potentially permit discovery of internal corporate documents.

The firearms company had convinced a lower court that the federal law prevented the families’ suit. But Connecticut’s high court said that the law had exceptions, and that one of them was meant for state consumer protection laws.

The families’ fight is not over, though. The case now returns to a local court in Bridgeport, Connecticut, which must determine from the evidence whether Remington is responsible.

An appeal is likely, as the shield for gunmakers provided by federal law now has a crack in it that other states could seek to leverage.

The National Rifle Association fought hard to get the immunity from Congress, saying it was needed to protect U.S. companies from costly and what they see as unfair litigation that blames them for the crimes of others. The organization did not respond to a request for comment.

Gun control groups were elated by the court’s ruling.

The decision is “a victory for the principle that no industry is above the law or above accountability,” said a statement from Adam Skaggs, chief counsel of the Giffords Law Center to Prevent Gun Violence.

The Connecticut court said that the suit, which seeks unspecified damages for each death, could go forward under that state’s Unfair Trade Practices Act, a statute aimed at harmful marketing, in this case marketing not of the weapon but rather its use as a potential tool for “offensive military style combat” by civilians, which is illegal.

Twenty first-grade children and six adults were killed and two others were wounded in the attack on Dec. 14, 2012. Lanza, who took his own life after the assault, shot and killed his mother, Nancy Lanza, in her home before targeting the school. The Bushmaster that Lanza used had been sold to his mother, according to court documents, and shared with Adam, who was 20 at the time of the killings.

“The number of lives lost in those 264 seconds was made possible by the shooter’s weapon of choice,” the plaintiffs said in their lawsuit. “Plaintiffs seek nothing more and nothing less than accountability” for Bushmaster’s choice to “continue profiting” from selling and marketing the weapon for civilian use, the suit said.

Among the allegations in the lawsuit was that the gun manufacturers and dealers knowingly marketed the weapon’s use to carry out deadly missions against “perceived enemies,” as the court said.

Such use of the Bushmaster or any weapon would be illegal, it added, and “Connecticut law does not permit advertisements that promote or encourage violent, criminal behavior.”

The federal law, called the Protection of Lawful Commerce in Arms Act, does immunize firearms makers from liability for crimes committed by people using their weapons, the court agreed. But it said that “we also conclude that Congress has not clearly manifested an intent to extinguish the traditional authority of our legislature and our courts to protect the people of Connecticut from the pernicious practices alleged in the present case.”

The dissenting justices said the majority, in this “national question of first impression,” was misreading the federal shield law.

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