NEW YORK (CNN) -- A federal jury in a landmark civil lawsuit Thursday found 15 handgun manufacturers negligent in their marketing and distribution practices.

Early in their sixth day of deliberations, the jury told Judge Jack Weinstein that one of them was holding out because he or she felt the verdict could "open the floodgates to lawsuits across the country."

In the closely watched case, the families of seven shooting victims in Brooklyn sued gun manufacturers for negligence for allegedly glutting certain markets with weapons, making it easier for criminals to obtain illegal guns.

The civil suit is the first to come to trial using the "negligent marketing" charge.

Weinstein told the jury to return to deliberations after he instructed them not to consider the effects of the verdict beyond "the evidence heard here and the law as I gave it."

The jury's revelation was a surprise to those watching the case. Late Wednesday, Weinstein warned the lawyers to prepare for a mistrial after the two men and nine women jurors called themselves deadlocked for a third time in two days.

But Thursday morning's note seemed to reflect a change in the mood of the jurors.

"After listening to the judge yesterday afternoon, 10 jurors decided to work together to reach a verdict," they said in the note. "One juror refused because he or she feels the verdict 'will open the floodgates of lawsuits across the country.' Could the judge address the importance of focusing on only the evidence in these seven cases?"

Plaintiffs' lawyers moved to remove the holdout juror, but the judge denied their motion. One defense lawyer complained that "the jury is horse-trading" based on its query Wednesday whether it should compromise on a verdict. The defense asked for a mistrial, but the judge also denied that motion.

During four weeks of testimony, the plaintiffs presented witnesses to support the theory that manufacturers send too many firearms to Southern states with weak handgun laws. The excess guns, they allege, end up in the hands of illegal traffickers who resell them in Northern states that have stricter laws.

The gun companies deny the accusation, saying they are being wrongly blamed for the criminal actions of individuals.

Among the defendants in the case are the No. 1 U.S. handgun maker, Smith & Wesson Corp. of Springfield, Massachusetts, a unit of British conglomerate Tomkins Plc, and Sturm, Ruger & Co. of Southport, Connecticut, the second-largest U.S. handgun maker.

The trial has unfolded as other municipalities have begun suing the gun industry for their costs related to violent crimes, with five suits already filed and more expected. The outcome of the Brooklyn case could influence whether local governments choose to use a similar "negligent marketing" theory.

Reuters contributed to this report.