The number of homicides committed in the District with untraceable, home-built “ghost guns” has risen to four, D.C. authorities said Friday as they proposed new legislation to prohibit kits used to make these new types of firearms.

One of the killings occurred in 2018, two last year and one this year, police said.

The numbers, up from a count police had provided earlier this week, added a new sense of urgency as the District’s mayor, police chief and forensics lab director said action was necessary to stop the flow of ghost guns into the city.

“The message we’re sending with this legislation is simple,” Mayor Muriel E. Bowser (D) said at a news conference. “If you assist with, participate in or profit from bloodshed in our community, we will hold you accountable.”

D.C. Police Chief Peter Newsham said detectives working with federal law enforcement agencies have at least two investigations in the works “where we have found some folks that are assembling these guns.” The chief declined to comment further on the cases. He would not say whether police believe the guns involved in those investigations were built in the District or elsewhere.

Assembling a gun out of parts obtained through a gun dealer or the Internet is largely legal in the United States. Four states have enacted laws regulating or prohibiting such guns. It is illegal to build a firearm in the District; the bill being proposed would make it illegal for residents to possess the kits.

Because ghost guns have no serial numbers, investigators lack the ability to trace them through owners and to the manufacturer. Instead, Newsham said, police have to rely on human intelligence to gather such information.

The officials spoke in front of two tables filled with ghost guns seized in the District. The number of such firearms found on the streets rose from three in 2017 to 116 in 2019. Another 38 have been seized this year.

While home-built guns vary in quality, they all resemble traditionally made semiautomatic handguns and rifles. Newsham said one ghost rifle was used in December “to almost assassinate two of our reserve officers at a 7-Eleven on Hawaii Avenue.”

He said making untraceable firearms that turn up in crimes “is not a hobby.” Added Bowser: “Ghost guns are real guns. . . . They will kill you.”

Mark Oliva, spokesman for the National Shooting Sports Foundation, the trade association of gunmakers, said the builders of ghost guns should be treated no differently from more traditional gun manufacturers. He said it is the people illegally distributing firearms — be they ghost guns or not — who should be targeted by law enforcement.

“There are already laws on the books covering this,” Oliva said. “If there are people selling firearms of this nature for the purpose of putting them in the hands of criminals, they are criminals, too, and they need to held accountable.”

But Oliva said building firearms is “a right protected under the Constitution.”

The types of ghost guns being used in the District, and in other cities, are 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, barrel and firing pin — are easily purchased online. Several companies that sell them have declined to comment on the subject to The Washington Post.

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 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 felons or domestic abusers, with instructions for finishing the guns easily available on the Internet.

The District is trying to close what it calls that loophole in the law by enacting legislation, to be proposed Tuesday to the D.C. Council, making it illegal for people in the city to possesses the 80 percent kits.

If passed, the emergency legislation would be in place for 90 days. Officials said they hope to enact a permanent law during that period.

Officials said the makers of the kits have stopped selling them in other states that enacted similar bans.

D.C. Council member Charles Allen (D-Ward 6), who is sponsoring the bill, said that three years ago, it was rare for police to come across a ghost gun. Now, he said, “it is a trend developing across the city.”