Dig beneath the top-level numbers in the Federal Bureau of Investigation’s report on 2016 crime statistics, and a striking pattern reveals itself. As homicide increased for the second year in a row, one category of weapons killed at a far greater rate than any other: handguns.

The FBI’s numbers show that the national homicide rate rose by 8.6 percent last year, following a 10.8 percent increase in 2015. As more Americans were killed, more of them were killed by the same means — 73 percent of 2016’s recorded homicides were committed with a gun, the highest share the FBI has ever reported.

And most of those gun homicides, as the graphic below shows, were shootings by handguns, which surged again last year following a bump in 2015.

Handguns have long been used in more murders than rifles and shotguns, comparatively speaking.

Factoring in the recent uptick, pistols were used in 64,567 homicides during the most recent 10-year stretch for which official FBI data is available:

It’s not surprising that handgun homicides account for most of the elevation in the national murder rates over the past two years. Following a sea change in the legal gun market, as gun companies cater to those who buy weapons to defend themselves from other people, the supply of handguns to the black market has also expanded.

After declining precipitously during the late 1990s, handgun manufacturing picked up again in the mid-2000s. The number of pistols produced rose from around 728,000 in 2004 to 4.5 million last year. No other kind of gun saw a bigger surge in output.

The black market for guns is intimately tied to the legitimate market. People who commit crimes with guns can get their hands on the weapons in several ways: In 32 states, gun owners can sell weapons in unregulated private sales that don’t require background checks. Straw buyers — legal purchasers who acquire a gun on behalf of a prohibited person — shop for pistols at licensed dealers immune from legal liability for the violent use of their wares. With guns allowed in more public spaces, theft is also an essential part of the criminal supply chain.

The connection between handgun manufacturing, homicides, and the black market is underscored by data on guns recovered at crime scenes. According to summaries of trace data by the Bureau of Alcohol, Tobacco, Firearms and Explosives, seven of the 10 calibers of gun found at crime scenes in 2015 are typically associated with handguns.

The other category of firearm homicides to rise last year was killings in which the type of gun used was left unspecified by local law enforcement. The FBI files those killings as “Firearm, type not stated,” and their numbers climbed to 3,077 last year, up from 1,587 in 2011.

The FBI would not comment on that increase.  Michael Maltz, a criminal justice expert, said he thought the percentage of homicides that lack data could be rising due to budget cuts that can adversely affect training and record keeping. “It would only take a couple major cities to stop reporting those details to have a pretty major effect on the FBI’s numbers,” Maltz said.

Michael German, a former FBI agent who now works at the Brennan Center for Justice, had another hypothesis: Criminals, like legal gun owners, increasingly favor 9mm, .40, and .45-caliber handguns, which pack more power than the smaller, cheaper guns more common in shootings through the 1990s. The trend could means more shootings that result in “pass through” wounds, in which the bullet enters one side of the victim and exits the opposite side, without becoming lodged in tissue or bone.

In such a scenario, forensics teams would have a harder time finding and identifying the bullet.

And “if there’s no round found, then you can’t say what kind of gun was used,” German said.