When Flint, Mich., announced in September that 68 A-15 style weapons collected in a gun buyback would be incinerated, the city cited its policy of never reselling firearms.

“Gun violence continues to cause enormous grief and trauma,’’ Mayor Sheldon Neeley said. “I will not allow our city government to profit from our community’s pain by reselling weapons that can be turned against Flint residents.’’

But Flint’s guns were not going to be melted down. Instead, they ended up at a private company that has collected millions of dollars taking firearms from police agencies, destroying a single piece of each weapon stamped with the serial number, and selling the rest as nearly complete gun kits. Buyers online can easily replace what’s missing and reconstitute the weapon.

Hundreds of towns and cities have turned to a growing industry that offers to destroy guns used in crimes, surrendered in buybacks, or replaced by police force upgrades. But these communities are in fact fueling a secondary arms market, where weapons slated for destruction are recycled into civilian hands, often with no background check required, according to interviews and a review of gun disposal contracts, patent records, and online listings for firearms parts.

Some public officials and gun safety advocates said they had no clue this was happening. The Rev. Chris Yaw, whose Episcopal church outside Detroit has sponsored buybacks with local officials, said in an interview that he was “aghast and appalled’’ when told by a reporter how the process works.

“It tells me that our society is set up really well for buying and selling guns,’’ he said, “but it’s not set up very well for disposing of them.’’

This examination of the gun disposal industry reveals a hidden aspect of the government’s role in promoting the proliferation of guns and a gun culture that has divided the country.

The industry relies on contracts with public agencies at the local, state, and federal levels and is subsidized by tax dollars and charitable donations that pay for buybacks. Governments arguably could be seen as complicit in bad outcomes — if a recycled assault weapon from Flint, for example, was later used in a deadly shooting — but it would be difficult to even know that; the salvaged gun parts typically would not include a serial number that could be traced.

A Missouri business called Gunbusters, which patented a “firearms pulverizer,’’ was responsible for dealing with the Flint weapons. The company says it has taken in more than 200,000 firearms over the past decade from about 950 police agencies around the country, from Baton Rouge, La., to St. Louis to Hartford, Vt.

At least a half-dozen other firms do similar work. LSC Destruction of Nevada says it has disposed of guns for police departments in Minneapolis and San Antonio, while New England Ballistic Services of Massachusetts has worked with Boston and towns in Rhode Island.

Gun auction websites have thousands of listings for parts kits and even complete firearms offered by firms that contract with law enforcement agencies to handle disposals. Gunbusters and its five licensees across the country, for example, recently averaged more than $90,000 a week in combined online sales of hundreds of disassembled guns from government clients.

This little-known but profitable corner of the firearms economy exists because the approved method of destroying a gun contains a loophole that has been exploited.

To be able to say a gun is destroyed, disposal companies crush or cut up a single piece that federal law classifies as a firearm: the receiver or frame that anchors the other components and contains the required serial number. The businesses can then sell the remaining parts as a kit: barrel, trigger, grip, slide, stock, springs — essentially the entire gun, minus the regulated piece.

Police agencies and disposal companies say they are following guidelines set by the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. While the guidelines, posted on the ATF website, show illustrations of whole guns being cut into pieces with an acetylene torch, they also say that an “acceptable method’’ is to destroy just the receiver or frame.

The companies, for their part, say that if public officials want the whole gun destroyed, they must pay for it.

“Our services are free for law enforcement agencies,’’ said Scott Reed, president of Gunbusters. “If we can’t cover our costs by selling parts, then we charge them.’’

Only about 2 percent of Gunbusters’ clients pay to have the full firearm destroyed, he said. Federal agencies, including the Secret Service, are among them.

Reed likened the recycling of parts to “organ donation,’’ allowing collectors to repair or maintain their firearms: “The people who are happiest with us are those who need parts for old guns that just aren’t made anymore.’’

But while the parts kits have legitimate uses, they could also further the spread of so-called ghost guns when paired with an untraceable receiver or frame, said Nicholas Suplina, a senior lawyer with Everytown for Gun Safety. The number of do-it-yourself ghost guns turning up in violent crimes has surged, made possible by unfinished components — prefabricated metal pieces that need welding and drilling — that are not serialized and often do not require a background check when purchased separately.