Twenty years ago Saturday, the Brady Handgun Violence Prevention Act became the law of the land. This groundbreaking gun-safety law requires background checks on all gun sales by licensed dealers.

Background checks on gun purchases work. The law has stopped more than 2 million convicted felons, domestic abusers and individuals with serious mental illnesses from purchasing firearms.

But the Brady Act has a big loophole: It does not require background checks on the estimated 40 percent of gun transfers made between private parties. This means anyone can purchase a firearm at a gun show or over the Internet without undergoing any sort of background check. Last year, an estimated 6.6 million firearms were transferred this way.

Critics have argued that expanding background checks to close this loophole is pointless because no criminal would consent to a background check. California law — which requires background checks on all gun sales except those between family members — proves otherwise.

This year, background checks in California have been conducted on more than 750,000 gun sales. Nearly 5,800 prospective gun purchasers were denied because they failed the background check. Of those 5,800 denials, 536 — or nearly 10 percent — involved sales between private parties.

Bottom line: So far this year, California’s law has stopped more than 500 criminals, domestic abusers and mentally ill persons from purchasing weapons.

California’s gun death rate has been cut by 56 percent over the past 20 years, according to the Law Center to Prevent Gun Violence, in part because background checks are required on these private sales. Our state’s system is working, and Californians are safer because background checks on private sales are mandatory.

This has proved true in other states as well. In states that require background checks for private handgun sales, 38 percent fewer women are shot to death by their partners and 39 percent fewer law enforcement officers are shot to death with handguns, according to an analysis conducted by Mayors Against Illegal Guns.

But only 16 states require background checks on these private transactions. In the other 34 states, private transfers happen every day with no questions asked.

The case for expanded background checks is more urgent now than ever before. The Internet makes it even easier for would-be gun buyers to acquire firearms without submitting to background checks.

A recent report by Mayors Against Illegal Guns examined a sample of individuals who sought to buy a firearm by posting a “want-to-buy” advertisement on one popular gun classifieds website. Of these prospective buyers, 1 in 30 had a criminal record that bars the person from owning a gun.

With an estimated 790,000 for-sale or trade ads posted over the course of a year — or more than 2,000 each day — more than 25,000 guns may be transferred to criminals each year through just this one website. These figures do not count guns transferred to people who are prohibited from possessing firearms for other reasons, such as mental illness.

A status quo where 40 percent of gun sales occur without background checks contributes to the plague of gun violence in this country and must not be allowed to continue. Two decades after the Brady Act was signed into law, it is time for Congress to expand our nation’s background check system to cover Internet, gun show and other private party sales.

We know background checks work. Let’s finish the job we started 20 years ago and finally make expanded background checks a reality in this country.

Dianne Feinstein is California’s senior U.S. Senator. She wrote this for this newspaper.