At least nine of those guns have now been found at crime scene and or with people wanted on warrants for violent offenses, D.C. Attorney General Brian Schwalb said. Many of the others are still unaccounted for.

"Our city is being flooded with illegal weapons," he said. "All three of these stores ignored the red flags."

Washington, D.C., has struggled with gun violence in recent years. The nation's capital saw its highest number of homicides in more than three decades last year, and more than 90% of those were carried out with firearms, the suit states.

"Many of us watch the news and we wonder where all these guns are coming from," said Maryland Attorney General Anthony Brown. "Now we have part of the answer."

In Washington, the supply of weapons is often fueled by people who buy guns for others who can't legally possess them, Schwalb said. About 95% of guns recovered in Washington, D.C., which has strict gun laws, originally come from nearby Maryland or Virginia, Schwalb said. While some of those are stolen weapons, more come from illegal straw sales, according to data about firearm trafficking investigations from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The suit seeks unspecified damages and court action to halt any future straw purchases.

The lawsuit is the first to be filed jointly and comes as cities and states around the country file civil suits against gun shops, including in New Jersey, Minnesota, Chicago and Philadelphia. Kansas City also settled a suit last year against a gun dealer accused of ignoring evidence that guns were being sold illegally.

Licensed firearm dealers do work with ATF to identify possible straw purchases, said Larry Keane, senior vice president at the National Shooting Sports Foundation, an industry group. Still, he said that warning signs may not always be obvious at busy stores, where a buyer might encounter different employees on different days.

"The focus should be on the actions of the criminal, not trying to scapegoat retailers who do their best every day to try to prevent straw purchasing," he said, pointing to a 2016 Justice Department survey of people in prison that found a relatively small number had gotten firearms from a retail source.

The new suit, filed with the gun safety group Everytown Law, accuses the Maryland-based stores of failing to respond to warning signs, including bulk purchasing and repetitive purchases.

The three stores sold a total of nearly three dozen similar weapons to Demetrius Minor over a seven-month period in 2021, the suit said. Nearly all were trafficked to others, including people who aren't legally allowed to buy firearms, the suit alleges. One gun, for example, was found in a D.C. hotel room along with an illegal large-capacity magazine and another was found at the home of a stabbing suspect, the suit says.

Minor pleaded guilty to one count of dealing in firearms without a license last year in a plea deal with prosecutors and was sentenced to 18 months in prison. An attorney who represented Minor could not immediately be reached for comment.

One store, Atlantic Guns, Inc., said it has "never and will never knowingly sell to someone who we have reason to believe is committing a straw purchase." Another, United Gun Shop, declined immediate comment, and the third, Engage Armament LLC, did not immediately respond.

The lawsuit seeks unspecified damages and court action to halt any future straw purchases.