ALBANY — A little more than a year ago, some law enforcement agencies across New York began encountering so-called "ghost guns" more frequently.

These untraceable firearms, including assault-style rifles and semiautomatic handguns, are increasingly finding their way into the hands of criminals, or anyone with the minimal skills needed to assemble them.

Thousands of kits used to build the guns are being sold annually across the nation by online mail-order companies, which exploit a loophole in federal and state gun control laws by providing "unfinished" hardware with the drill bits and instructions — including video tutorials — needed to make a fully functioning firearm.

What is a ghost gun, and it is legal?

The increase in self-manufactured guns has taken place as New York's political leaders, including U.S. Sen. Charles E. Schumer, have focused their legislative efforts largely on strengthening background checks for firearms permits and cutting off easy access to certain assault weapons.

Last year in the Syracuse area, police agencies seized 15 ghost guns between July 18 and the end of December — including 10 semiautomatic handguns, three semiautomatic rifles, and an illegally modified fully automatic rifle. This year, those agencies have recovered an additional six ghost guns, five of which were semiautomatic handguns, according to data provided by the Onondaga County district attorney's office.

"We've had them used in exchanges of gunfire between rival gangs. Car stops. A suspect in a murder had a second gun that was one of these," said Shaun M. Chase, the county's chief assistant district attorney. "It's definitely a newer phenomena."

On Thursday, Onondaga County District Attorney William Fitzpatrick held a news conference about the proliferation of ghost guns in that region. The news conference was held two days after Fitzpatrick's office was contacted about the trend by the Times Union.

They have been dubbed ghost guns because the self-manufactured weapons have no serial numbers and are unregistered. Their owners often lack the proper state permits or have not undergone the federal background check needed to legally possess them. And in many instances, they are being illegally resold to convicted felons.

In February, State Police and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) charged 38-year-old Gregg Marinelli of Ulster County, who was a police sergeant for the New York City Department of Environmental Protection, with illegally manufacturing dozens of handguns and assault rifles, including one fully automatic rifle.

Police said many of the firearms made by Marinelli were ghost guns, which he had sold to members of "outlaw motorcycle groups" and convicted felons — sometimes delivering the guns in his marked police car.

"It is definitely an emerging trend for law enforcement to deal with," said Matthew T. Fleming, a spokesman for the New York ATF office.

Despite the proliferation of the underground firearms, there is still confusion among some law enforcement officials who may wrongly believe the term "ghost guns" refer only to the high-tech plastic firearms that can be produced using expensive 3D printers. Those firearms, which are undetectable by standard security screening devices, are rarely encountered by law enforcement despite swift action from New York lawmakers this year to outlaw them.

"3D guns and improperly stored firearms pose an enormous risk to our children, and today we're addressing both dangers head-on to keep our families safe," Gov. Andrew M. Cuomo said in July when he signed the bill outlawing 3D-printed guns.

But State Police and many other law enforcement agencies said they have not seized or encountered so-called 3D guns in criminal investigations.

Separate legislation that would have banned the more prevalent practice of online sales of the self-manufactured firearms was quietly shelved by state lawmakers in May, and did not reach the floor of the Senate or Assembly for a vote. That legislation had specifically targeted the type of firearms being sold by the online retailers.

Federal law allows someone to make a firearm for personal use and does not require them to obtain a serial number or to register the weapon. But New York laws require anyone in possession of certain firearms, especially handguns, to obtain a permit and register the gun.

Still, ghost guns are often obtained by individuals who are prohibited from legally possessing a firearm or seeking to use them for criminal purposes. They cannot be sold or transferred unless the person who owns them has a federal firearms license.

The online companies manufacturing the gun kits market them as "80 percent finished," which puts them just below the definition of a firearm under the federal Gun Control Act of 1968. That loophole has led law enforcement officials to dub those firearms "80 percenters."

"A difference of two holes makes it a gun or not," a law enforcement source said.

Brian Olesen, owner of Capital Gun Group, which operates 10 gun stores in the region, said there has been a significant uptick in ghost guns recently across upstate New York.

Gun store workers said it's also becoming more common for individuals with no pistol permit to show up at their shops with unregistered handguns. Often, they are seeking help finishing their unregistered weapons because they assembled them incorrectly, or lack the tools and expertise needed to drill holes in the parts that make them capable of firing.

Olesen said the state's 2013 SAFE Act gun control law "ignored the fact that we are awash in ghost guns that are unserialized and unregistered. These guns four years ago were unheard of. Right now, they’re everywhere."

In November 2017, a California shooter who killed five people and injured 10 — including seven children — had assembled his own AR-15 assault-style rifles from a ghost-gun kit before embarking on his deadly rampage. The shooter had failed an earlier gun background check.

Last month, a convicted felon used a AR-15-style ghost rifle when he opened fire on police along a Los Angeles freeway, killing one officer and wounding two others. An ATF official in Los Angeles told the Los Angeles Times after the shooting that about one-third of the firearms being seized by law enforcement in southern California are ghost guns, "and that is expected to grow."

A federal judge in Florida last year sentenced an Orlando man, Hector Luis Santiago-Jorge, to five years in prison after he admitted assembling and distributing more than 200 guns — mostly AR-15 assault-style pistols — that were delivered to his mobile home over an 11-month period.

Federal agents who searched a shed adjacent to Santiago-Jorge's residence, along South Orange Blossom Trail about 13 miles from Walt Disney World, said it contained ammunition, drills and equipment used to assemble the guns. Inside his pickup truck, they recovered 10 disassembled AR-15 assault-style pistols still in delivery boxes.

More than 190 of the weapons that had been built by Santiago-Jorge, who was already a convicted felon at the time of his arrest, were never recovered. Law enforcement authorities said he claimed to have distributed them in Puerto Rico.

Erie County District Attorney John J. Flynn said ghost guns have not been as prevalent in that area, but last week Buffalo police encountered their first case and arrested a man who had constructed a homemade rifle.

"There's no doubt about it that they're probably here," Flynn said.