The gunman who killed 12 people at a Virginia Beach municipal building on May 31 was armed with two handguns and several high-capacity magazines. His weaponry enabled him to engage in a “long-term gun battle” with police before he was fatally shot.

In the wake of mass shootings, gun violence prevention advocates and politicians often focus on the role played by so-called assault weapons like AR-style rifles. But a growing body of research suggests that magazine capacity, more than weapon type, may sharply increase death tolls by allowing shooters to fire more rounds before they are stopped.

“Over half of high-fatality shootings in past 30 years involved high-capacity magazines,” said Louis Klarevas, the author of the book “Rampage Nation,” adding that larger magazines “correlate with higher death tolls.” If a shooter only has magazines that hold 10 rounds or fewer, he will have to reload more frequently, giving bystanders more opportunities to flee, shelter, or intervene.

Virginia, like most states, places no limits on magazine capacity, and an effort to enact such a restriction earlier this year failed in the face of GOP opposition. However, the available data suggests that when the devices are regulated, they’re used less often in crimes.

A federal assault weapons ban, from 1994 to 2004, prohibited the purchase and sale of magazines capable of holding more than 10 rounds. An analysis by The Washington Post of Virginia State Police records found that seizures of high-capacity magazines steadily declined when the federal law was in effect. In 1997, police in the state recovered 944 handguns with high-capacity magazines, but by 2004 the number had fallen to just 452. However, once the ban expired, police saw the trend reverse itself: By 2009, Virginia law enforcement seized 986 such handguns.

”The federal ban had preventative value,” said Christopher Koper, a criminologist at George Mason University. “It helped prevent crimes committed with the magazines. And after the ban expired, we saw an increase in attacks with these magazines, which tend to have more deaths and injuries.”

Koper is the co-author of a paper that examined the efficacy of state laws to limit magazine capacity. He found similar results. To conduct their research, Koper and his colleagues analyzed gun seizure data from 10 cities around the country for the prevalence of high-capacity magazines.

They found that of all the cities, Syracuse, New York — located in a state that has had a strict limit on magazine capacity since the early 2000s — had the lowest percentage of crime guns equipped with the devices. Only 14.6 percent of weapons seized by police in Syracuse had high-capacity magazines, the lowest of any city studied. Meanwhile, Seattle and Kansas City, Missouri, cities located in states without limits on magazine size, recorded the most recoveries of guns with high-capacity magazines, at 36.2 percent.

Moreover, once the federal law expired, police encountered more of the formerly banned high-capacity magazines on the street. Guns equipped with high-capacity magazines “have grown substantially as a share of crime guns since the expiration of the federal ban,” Koper and his co-authors wrote.

Still, Republican lawmakers around the country have generally opposed efforts to regulate the devices. In Virginia, the GOP-controlled General Assembly has ensured that the state has not created its own version of the lapsed federal ban. In January, Republicans in the state Senate stopped a bill to limit magazine capacity from advancing out of committee.

The day after the Virginia Beach shooting, Republican state Senate Majority Leader Thomas Norment told The Washington Post it was too soon to bring up the magazine ban his party had quashed earlier this year, calling such efforts “offensive, disrespectful and tasteless.”