NEW YORK — The Connecticut Supreme Court dealt a major blow to the firearms industry Thursday, clearing the way for a lawsuit against the companies that manufactured and sold the semi-automatic rifle used by the gunman in the massacre at Sandy Hook Elementary School.

The lawsuit mounted a direct challenge to the immunity that Congress granted gun companies to shield them from litigation when their weapons are used in a crime. The ruling allows the case, brought by victims’ families, to maneuver around the federal shield, creating a potential opening to bring claims to trial and hold the companies, including Remington, which made the rifle, liable for the attack.

The decision represents a significant development in the long-running battle between gun-control advocates and the gun lobby. And it stands to have wider ramifications, experts said, by charting a possible legal road map for victims’ relatives and survivors from other mass shootings who want to sue gun companies.

In the lawsuit, the families seized upon the marketing for the AR-15-style Bushmaster used in the 2012 attack, which invoked the violence of combat and used slogans like “Consider your man card reissued.’’

Lawyers for the families argued that those messages reflected a deliberate effort to appeal to troubled young men like Adam Lanza, the 20-year-old who charged into the elementary school and killed 26 people, including 20 first-graders. The attack traumatized the nation and made Newtown, Conn., the town where it happened, a rallying point in the debate over gun violence.

In the 4-3 ruling, the justices agreed with a lower court judge’s decision to dismiss most of the claims raised by the families, but also found that the sweeping federal protections did not prevent the families from bringing a lawsuit based on claims of wrongful marketing. The court ruled that the case can move ahead based on a state law regarding unfair trade practices.

In the majority opinion, the justices wrote that “it falls to a jury to decide whether the promotional schemes alleged in the present case rise to the level of illegal trade practices and whether fault for the tragedy can be laid at their feet.’’

“I am thrilled and tremendously grateful,’’ said Nicole Hockley, whose 6-year-old son Dylan was killed in his first-grade classroom. “No one has blanket immunity. There are consequences. We want our day in court to see why they do this this way, and what needs to change.’’

Victims’ families have also had successes recently in lawsuits against Alex Jones, the far-right provocateur who spread bogus claims about the shooting, including that the families were actors involved in a wider plot to confiscate firearms.

The families sought legal action after they received death threats and were targeted for harassment.

As for the decision issued Thursday, lawyers for Remington, as well as other gun companies named in the suit, did not immediately respond to requests for comment.

“The majority’s decision today is at odds with all other state and federal appellate courts that have interpreted the scope’’ of the federal protections, the National Shooting Sports Foundation, a trade association representing the firearm industry, said in a statement.

The families faced long odds as they pursued a novel strategy to find a route around the federal protections and will confront major hurdles as the case proceeds. Their hope was to bring the case to trial, which could force gun companies to turn over internal communications that they have fiercely fought to keep private and provide a revealing and possibly damaging glimpse into how the industry operates.

Nora Freeman Engstrom, a law professor at Stanford University and an author of an amicus brief signed by law professors in the case, said the ruling showed that the federal protections were “not impenetrable.’’

“This is not the end,’’ she said, adding, “Any path for plaintiffs will be long and strewn with obstacles. But this opinion suggests there may well be a road, which before was unclear.’’

The ruling comes as yet another twist in the lawsuit’s circuitous path through the courts, one that continued far longer than many, including legal experts, had initially expected. “This decision was a long time in coming, but it was more than worth the wait,’’ said Joshua D. Koskoff, a lawyer for the families.

The ruling had been delayed after Remington filed for bankruptcy protection last year as sales fell and debts mounted.

The lawsuit, brought by family members of nine people who were killed and a teacher who was shot and survived, was originally filed in 2014, then moved to federal court, where a judge ordered that it be returned to the state level.

The families were given a glimmer of hope when a state judge permitted the case to approach a trial before she ultimately dismissed it. She found the claims fell “squarely within the broad immunity’’ federal law offers. In 2005, Congress restricted suits against gun sellers by granting immunity from blame if one of their products is used in a crime. Lawmakers cited a need to foil what they described as predatory and political litigation.

The law allows exceptions for sale and marketing practices that violate state or federal laws and instances of negligent entrustment, in which a gun is carelessly given or sold to a person posing a high risk of misusing it. The families pushed to broaden the scope to include Remington.