The attorneys general of D.C., Maryland and Virginia are supporting a federal lawsuit seeking to have the Bureau of Alcohol, Tobacco, Firearms and Explosives regulate the widely sold parts of homemade “ghost guns” as firearms, in an attempt to stop the steadily increasing use of the untraceable firearms in crimes across the country.

The parts for a “ghost gun” can be ordered easily on the Internet, and instructional videos show how an “80 percent lower” part of a gun can be milled and drilled into a fully functional “100 percent” lower. That piece then can be combined with the unregulated upper parts — the trigger, the barrel, the firing pin — to make a gun that has no serial number and requires no background check.

After D.C. police began recovering increasing numbers of the guns, often connected to crimes, the District this year passed emergency legislation banning the kits used to make ghost guns. D.C. Attorney General Karl A. Racine also filed a consumer protection lawsuit in June against one of the largest makers of the gun parts, Polymer80, which is pending.

In 2017, D.C. police found three such guns, and by last year that number had risen to 116. In those three years, Racine said, nine ghost guns were reportedly involved in homicides. This year, local and federal police had recovered 282 ghost guns in the city as of Dec. 17, according to the District’s Department of Forensic Sciences.

The local attorneys general joined 16 other state attorneys general in a brief supporting a federal suit filed in the Southern District of New York by the cities of Syracuse, N.Y., San Jose, Chicago and Columbia, S.C., and the pro-gun-control group Everytown for Gun Safety, against ATF, the Justice Department and the U.S. attorney general. The suit targets several “interpretive rulings” issued by ATF to gun parts manufacturers in recent years, stating that the unfinished lower and upper parts of guns are not, themselves, firearms.

The ATF’s website says that “items such as receiver blanks, ‘castings’ or ‘machined bodies’ in which the fire-control cavity area is completely solid and un-machined have not reached the ‘stage of manufacture’ which would result in the classification of a firearm” under federal law.

The manufacturers then posted the ATF rulings on their websites to reassure customers that the parts they are buying are legal. Such rulings “encouraged and emboldened the ghost gun industry to sell its products nationwide,” even in states that have banned them, the filing said. Since 2015, D.C. and six states have enacted ghost-gun-specific statutes, the attorneys general said in their amicus brief.

There are about 80 online sellers of partially finished lower frames (for handguns) and receivers (for long guns). The ATF said last year that about 30 percent of guns recovered in California had no serial numbers. Ghost guns were used in mass shootings at Santa Monica College in 2013 and in Rancho Tehama, Calif., in 2017, made by men who were prohibited from legally buying weapons.

“The ATF’s reckless interpretation of the law and lack of regulation could lead to more untraceable guns on our streets, potentially putting Virginians and their families at risk,” Virginia Attorney General Mark R. Herring said in a news release.

The ATF has not yet responded to the suit in New York. But in a similar suit filed by the state of California, two fathers of ghost-gun shooting victims and the Giffords Law Center to Prevent Gun Violence, the agency defended its interpretation of the federal Gun Control Act of 1968. The bureau argued that “a receiver blank may not ‘readily be converted’ into a firearm,” because it requires numerous milling and metalworking steps, and “a working gun cannot be produced ‘without difficulty.’ ”

The unfinished gun parts are “therefore not a ‘firearm’ within the meaning of the statute,” Justice Department attorneys said last month in the California case.

The Gun Control Act defines a firearm as any weapon that is “designed to or may readily be converted to expel a projectile by the action of an explosive.” Kathleen Konopka, the D.C. deputy attorney general for public advocacy, said in an interview that many states patterned their laws after the federal act. “To our surprise, ATF has now interpreted this otherwise. We feel that’s a misinterpretation of federal and state statutes,” she said.

Making a gun at home is legal, and has long been done by firearms enthusiasts. And the advent of 3-D printers has made the process easier, though experts say those guns made with plastic do not function well for long, and D.C. police have found few of them.

But only licensed firearms dealers may sell guns, so making a gun with no serial number — or buying one from someone who did — is attractive to criminals. There is no background check, in which people with criminal records or domestic violence or mental health issues can be prevented from a purchase, and no serial number makes it much harder for police to track the history of a gun and determine who bought it or who owned it after it was used in a crime.

“The fact that they’re not regulated,” Konopka said, “has really added to the crime in the city and decreased the ability to solve those crimes.”

“The ATF’s interpretation of the law,” Maryland Attorney General Brian E. Frosh said in a release, “allows criminals who cannot pass a background check to obtain untraceable firearms. It is a flat-out danger to law abiding Americans.”

This article has been updated to clarify how a ghost gun is made.