MENLO PARK, Calif. (AP) — Facebook will soon rely on centuries-old technology to try to prevent foreign meddling in U.S. elections: the post office.

Baffled in 2016 by Russian agents who bought ads to sway the U.S. presidential campaign, Facebook’s global politics and government outreach director, Katie Harbath, told a meeting of the National Association of Secretaries of State in Washington on Saturday that the company would send postcards to potential buyers of political ads to confirm they reside in the U.S.

The recipient would then have to enter a code in Facebook to continue buying the ad. The method will first apply to ads that name candidates ahead of the midterm elections in November, said Facebook spokesman Andy Stone.

The plan was unveiled a day after special counsel Robert Mueller charged 13 Russians with interfering in the presidential election. Mueller’s indictment described how Russian agents stole social security numbers and other information from real Americans and used them to create bank and PayPal accounts in order to buy online ads. Agents also recruited Americans to do things such as hold up signs at rallies organized to create content for Russian-created social media posts.

Facebook uncovered some 3,000 Russian-linked ads on Facebook and Instagram bought before and after the November 2016 election that it says may have been seen by as many as 150 million users. But ads were only part of the problem, as the Mueller indictments say that Russian agents also set up fake pages with names such as “Secured Borders,” ”Blacktivist” and “United Muslims of America” that had hundreds of thousands of followers.

Facebook did not say how the new postcard method of verification would prevent foreign agents from setting up local mailing addresses and hiring people in the U.S. to check them. But Stone said the method was “one piece of a much larger effort to address foreign electoral influence on our platform.”

Facebook’s efforts largely center around verifying people on the platform are who they say they are. To catch duplicitous ad-buyers, for instance, it is now testing out in Canada a system that allows people to see which ads are being bought by a Facebook page — say, a candidate’s — even if the person checking the ad is not in the group to whom the ad was intended to be shown.

Stone said Facebook was also able to detect and remove “tens of thousands” of fake Facebook pages in advance of French, German and British elections last year using improved machine learning techniques.

The company has said it would double the number of people working on its safety and security team to 20,000 this year and add 1,000 people to review advertising content.