Tens of thousands of guns will be sold through online portals in Washington state this year, and none of the buyers will be subject to background checks. That’s because of a loophole in federal law that allows unlicensed dealers to make sales online without conducting criminal records checks on those who purchase their firearms.

A new study suggests a significant portion of those buyers — at least 10 percent — would fail a background check conducted by a licensed dealer.

The study, conducted by the pro-gun control group Everytown For Gun Safety, examined sales through several Web sites that serve as online portals for firearms dealers. Sites like Armslist, Northwest Firearms, Outdoors Trader, Washington Gun Trader and Gun Listings all allow sellers to list their weapons for sale, and buyers to post notices searching for specific kinds of firearms.

Those sites featured advertisements for nearly 17,000 weapons during a recent five-month stretch, Everytown researchers found. At the same time, another 1,164 buyers posted advertisements looking for weapons. When the group matched public records with any identifying information the buyers posted, they found almost 10 percent — eight out of 81 identifiable buyers — would be barred from buying a gun from a licensed dealer.

Federal law prohibits anyone convicted of a felony or domestic violence or anyone deemed severely mentally ill from possessing a firearm. That law prevents only a small number of people from owning a weapon: In 2013, gun dealers in Washington state conducted 561,000 background checks and denied only 3,600 people.

Among the identifiable buyers who sought weapons through online portals in Washington, several had been convicted of a domestic violence crime or were under a restraining order. Others were convicted felons, with rap sheets ranging from car theft to rape, burglary and robbery.

“[T]he background check system is effectively preventing criminals from obtaining guns at licensed gun dealers — but unlicensed sellers who offer an open door for acquiring guns without background checks are attracting them instead,” the authors of the Everytown report concluded.

The prevalence of online sales, which are excluded from background check requirements, has shown up in licensed dealer sales: Since 2003, the number of gun sales denied because of a failed background check dropped by more than half.

Everytown is among the groups backing Initiative 594, a measure on the ballot this fall that would require background checks on the vast majority of gun transfers in Washington state, including online and at gun sales. Gun-rights activists oppose the initiative, which they say would unduly burden gun owners who try to transfer weapons legitimately, even to family members.

Public polling shows I-594 passing by wide margins. But gun-rights advocates have their own alternative, Initiative 591, which would prohibit the state from enforcing background check measures that go beyond federal law. Some polls show that measure passing as well, leaving open the prospect of significant litigation after the election if both initiatives succeed.