Residents of Nevada voted in November to require background checks for most private gun sales, closing a loophole in federal law. But nearly a year after the ballot measure passed, these checks aren’t being made because state officials have claimed, with little evidence, that there’s no way to carry out the policy.

Such background checks might not have prevented the massacre in Las Vegas on Sunday night; the gunman, Stephen Paddock, is not known to have a significant criminal record and appears to have bought at least some of his guns from dealers required to vet buyers.

But it has been shown that requiring checks on private sales of guns, such as at gun shows and over the internet, makes it harder for felons and other prohibited people, like those who have been committed to mental institutions and drug addicts, to buy firearms. It is a common-sense gun-safety policy that most Americans and most gun owners have long supported. Even as Congress, cowed by the National Rifle Association’s opposition, has refused to act to limit the danger of unexamined sales, 19 states, including Nevada, have tightened background checks on handgun sales and 13 states, including Nevada, have tightened them for handgun and long gun sales.

After Gov. Brian Sandoval, a Republican, vetoed legislation to tighten background checks in Nevada in 2013, public interest groups like Everytown for Gun Safety took the matter directly to the state’s voters in November through a ballot initiative known as Question 1. The N.R.A. spent millions of dollars to defeat the proposal; its “no” campaign had the support of the state’s attorney general, Adam Laxalt, and many sheriffs. Yet, a narrow majority of people voted for Question 1, which had the support of police unions, health groups and others.