U.S. Sen. Tim Kaine commemorated the eighth anniversary of the Virginia Tech shootings that left 32 dead with a call for stronger gun laws.

"It’s estimated that 40 percent of all the guns that are sold in the United States occur with no background records check," Kaine, D-Va., said during an April 16 floor speech.

We asked Kaine’s office for the source of the statistic. Sarah Peck, his press secretary, pointed to a 1997 study by the U.S. National Institute of Justice that looked at gun owners’ responses to a 1994 national survey that asked them how they acquired their weapons. The researchers concluded 60 percent of the guns were bought from federally licensed firearm dealers and therefore subject to background checks.

Federal licensed sellers have to conduct background checks before sales, individual private sellers and gift givers do not. The co-authors of the study had released a more detailed report on their gun owners’ survey in 1996, estimating that between 30-40 percent of all guns transactions occurred through an "off-the-books" transfer in which a background check isn’t required.

The 40 percent figure has been long offered by gun control advocates as evidence that the nation needs tighter background check laws. But there’s been much debate about the research, conducted by Philip Cook of Duke University and Jens Ludwig of the University of Chicago, including:

The 1994 survey was conducted eight months after the Brady Act went into effect, mandating background checks on individuals seeking to buy firearms from federally licensed dealers. Survey participants were asked about gun their acquisitions going back two years.Some of the participants made gun purchases prior to the Brady Act, when they were not required to undergo federal background checks.

Only a small group of gun owners -- 251 people -- answered the survey question about the origin of their weapons. Some of the gun owners weren’t sure how they’d gotten their guns, answering "probably" or "probably not" on whether they got the gun from a licensed firearm dealer.

The 40 percent figure included transactions when a firearm was not sold but, for example, inherited or given as a gift. When the Fact Checker at The Washington Post asked Ludwig in 2013 to boil down the 1994 data, the researcher said that between 14 to 22 percent of the guns were purchased without background checks.

The survey is almost 21 years old and begs a question: What’s the modern-day percentage of guns sold without a background check? Cook pointed us to an editorial he and Ludwig wrote about the 40 percent statistic for the National Review in the aftermath of the December 2012 shootings at Sandy Hook Elementary School in Newtown, Conn.

"We’ve been following the give-and-take with some interest because our research is the original source of that 40-percent statistic," they wrote. "Our views may come as a surprise: First, we don’t know the current percentage -- nor does anyone else."

Ludwig and Cook said their national survey had not been replicated.

Updated data doesn’t exist, in large part, because federal funding for gun studies dried up two decades ago. President Barack Obama in 2013 signed an executive order to get the Centers for Disease Control and Prevention and other agencies to reexamine gun violence. The research is in progress.

There’s debate among researchers about whether the two-decade old figure on gun purchases without background checks remains reliable. Some, like Jay Corzine, a sociology professor at the University of Central Florida, say it is.

"That 40 percent without a background check is a pretty good estimate," said Corzine, who has researched crime prevention and the role of firearms in violence. "I have no quarrel with it."

The Center for American Progress, a liberal think tank, is less certain. The think tank said in a 2013 report that the figure has "clear limitations" given its age, and the fact that it comes from a survey of a small group of gun owners.

"Proponents of universal background checks, including this report’s authors, would be well served by acknowledging the limitations of the statistic," the center wrote.

Our ruling

Kaine says "it’s estimated that 40 percent of all the guns that are sold in the United States occur with no background records check."

His source is a small-sample survey that’s almost 21 years old. It counted some firearms that were purchased before a law went into effect requiring federal background checks for many gun sales.

The 1994 survey measured gun transactions, which included firearms that were sold and those that were given as gifts or inheritances. When the results were adjusted to guns sold -- the term Kaine uses -- only 14 to 22 percent of the firearms were sold without background checks.

The researchers say they "don’t know" if their findings are relevant today. There’s no updated national study.

Kaine did not acknowledge these points. So we rate his statement Mostly False.