The number of untraceable “ghost guns” built from kits and seized by police has begun to surge in the District and some other areas nationwide, raising concerns that firearm traffickers have found a new way to bypass background checks and pour more weapons into cities struggling with violence.

D.C. police said such guns were used in three killings in the city in recent years. District officers last year took 116 ghost guns off the streets, compared with just three in 2017.

Police in Philadelphia, Baltimore and suburban Maryland also said they are seeing more of the weapons, even as authorities in other big cities said they have yet to recover a single ghost gun.

The kits can be purchased without the background checks and other requirements needed to buy fully operational guns. Assembling a gun out of parts obtained through a gun dealer or the Internet is largely legal in the United States, and results in a weapon with no serial number or traceable link to a gun manufacturer: Thus, a “ghost gun.”

Four states have enacted laws regulating or prohibiting such guns, including California, which has endured three mass shootings by people using ghost guns.

On Friday, D.C. Mayor Muriel E. Bowser (D) is scheduled to announce emergency legislation to ban from the city the kits and parts that are used to make ghost guns.

“As Congress refuses to pass common-sense gun reform,” Bowser said, “we are going to do everything we can to strengthen our laws. By enacting our own laws on ghost guns, we are sending a clear message that if you use, buy or sell a ghost gun in D.C., you will be held accountable.”

Gun manufacturers don’t see the need to regulate ghost guns themselves, and say law enforcement should focus on those who deal them or use them illegally. “There are lots of individuals who like to make their own guns, which is perfectly legal,” said Larry Keane, senior vice president and general counsel of the National Shooting Sports Foundation, the trade association of gunmakers.

Keane noted, “It is already a crime under federal law to illegally deal in firearms” as a business, “whether they’re serialized or unserialized. If you don’t have a [federal firearms] license, you’re a criminal, and the industry wants to see those people stopped and prosecuted.”

Jenifer Smith, the director of the D.C. Department of Forensic Services, which runs the crime lab, said ghost guns are marketed “to keep them off the government books.” She added, “So the fact that they’re ultimately winding up in the hands of people who want to commit crimes with them isn’t necessarily surprising.”

In the summer of 2018, a ghost gun was used to kill a 25-year-old man during a shootout in Northeast Washington that left 32 shell casings at the scene, D.C. police said. Last year, the weapons were used in two more fatal shootings in troubled city neighborhoods.

Authorities also said a ghost gun resembling an AR-15 assault rifle was used to shoot at two volunteer reserve officers in Northeast Washington in December. The officers were pinned down by gunfire coming from a man hidden in the woods near Catholic University. Police said .223-caliber rounds struck their cruiser’s front bumper and the driver’s side door, though neither officer was struck.

D.C. Police Chief Peter Newsham said investigators have been unable to learn how the man obtained or built the rifle, key information the District tries to collect on every illegal firearm found in the city to understand and combat the influx of illegal weapons. Without serial numbers, Newsham said, “you have to get it through human intelligence. That’s the hard part.”

There are two types of ghost guns: those made out of plastic with 3-D printers, and those made with kits that supply 80 percent of the gun — called the frame for handguns or the receiver for long guns — already cut out of metal or polymer. Some minor drilling and milling is needed to add the top 20 percent. Both the “80 percent lowers” and the 20 percent remainder — the trigger, the barrel and firing pin — are easily purchased online from companies such as Polymer80, 80% Lower and U.S. Rifle. Those companies declined to speak about the weapons kits.

Blueprints for making 3-D guns, which would be hard to detect by metal detectors because they don’t contain metal, have circulated on the Internet and are the subject of ongoing litigation. Experts said those guns have not proved reliable in firing more than a handful of shots before collapsing.

A survey of numerous police departments by The Washington Post found that none had recovered a “3-D gun,” only the 80 percent lowers. The Bureau of Alcohol, Tobacco, Firearms and Explosives has ruled that the 80 percent lower is not a firearm, because it is unfinished and cannot itself fire a projectile. So there is no requirement that people buying the receivers undergo a background check, making them readily available to individuals such as convicted felons or domestic abusers, with instructions for finishing the guns easily available on the Internet.

In December, a man in Syracuse, N.Y., shot his 6-year-old nephew in the back with a ghost gun, and then was killed by police. Authorities in central New York then revealed they had recovered 24 ghost guns in recent months. At a news conference this month, Sen. Charles E. Schumer (D-N.Y.) called on the Justice Department and ATF to immediately amend the current policy to define the 80 percent lowers as firearms, requiring sellers to register their parts and purchasers to undergo background checks.

“Given the nature of the problem and the ability to ship these things across state lines,” said David Pucino, a staff attorney at the Giffords Law Center to Prevent Gun Violence, “the only truly comprehensive solution has to come at the federal level. . . . With 80 percent lowers, with minimal amounts of work, you can receive a firearm with no serial number and no way to check its origin. The promotional advertising makes it clear this market isn’t for hobbyists.”

“It’s something that we’re continuing to look at,” said Ashan Benedict, special agent in charge of the Washington field division of ATF. He noted that many gun laws date to the 1930s. “When it comes down to regulation, we’re going to continue looking at the laws, which were made 50 to 60 years ago, to see how they can be applied today.” He said ATF does not do nationwide tracking of ghost guns.

Keane said the government’s attention should be on anyone who commits a crime using the guns.

“If the gun is misused, the focus should be on the conduct, not the object,” he said.

California has been hit hard by individual instances of ghost gun violence. In 2013, a man who had been prohibited by California officials from buying a gun because of mental health issues constructed a .223-caliber AR-15-type rifle and killed six people at Santa Monica College.

In 2017, a man with a restraining order used two semiautomatic ghost rifles to kill five people in Rancho Tehama, Calif. And last year a teenager used a self-made .45-caliber pistol to kill two students at a high school in Santa Clarita, Calif.

California has not outlawed the parts for a gun, but in 2016 it required those who do make a gun to obtain a serial number from the state, place it on the gun and provide information about the owner to the state.

In 2018, New Jersey prohibited the purchase of gun parts for an unserialized gun, and authorities believe gun traffickers there are now having their parts shipped to Pennsylvania, which has not outlawed the 80 percent lowers. Last year, Connecticut and Washington state both passed laws restricting ghost guns.

A survey of police departments found surprising variations among cities and regions in how often the guns have turned up. Police officials in New York City, Chicago, Houston and Atlanta all said they had no reports of ghost guns, and Los Angeles said it had some but no exact count.

But Philadelphia police said they had recovered 118 ghost guns since the beginning of last year. Police in Prince George’s County, Md., have found 63 ghost guns since the start of 2019, and Montgomery County, Md., has recovered 60 since the beginning of 2017. Baltimore police said they don’t track those firearms, though a spokesman said they have seen “a major uptick in ghost gun seizures over the last year.”

In the District, the mayor plans to ask the D.C. Council to approve emergency legislation banning ghost gun kits. That allows Bowser to sign the bill into law without the more cumbersome reviews that can take months. The provision would last just 90 days, but officials said they hoped to get a permanent law passed in that period.

The District’s deputy mayor for public safety, Kevin Donahue, said these new types of firearms are “a workaround to many years of hard work that legislative agencies have done to regulate guns in a constitutional manner.” The District already prohibits people from building guns; the new proposal would bar the shipment of the kits and other gun parts into the city.

Donahue said that because the federal government does not consider the firearm kits to be guns, the District has “the flexibility and freedom to prohibit their use because they are not an object the Second Amendment applies to.”

Newsham said the ghost guns found by his officers in the District have been in “varying degrees of quality. Some are better than others — put together more professionally.”

Smith said it’s possible many of the ghost guns found in the District were made by one or a handful of people. She noted that the better-built weapons require high-speed drill bits or even ­expensive milling operations. But in the end, building a fully functional assault rifle from a kit is as simple, Smith said, as “taking a frame and drilling holes into it and putting the pieces together.”

Ghost guns accounted for 5 percent of the roughly 1,900 guns seized in the District in 2019, but they are more than 8 percent of the seizures so far this year, city officials said. District police said they found 31 ghost guns in the first six weeks of 2020.

Erin Cox contributed to this report.