During the 2016 presidential campaign, President Donald Trump’s operatives bragged to the press that they tried to dissuade African Americans from voting by targeting them with Facebook posts titled “Hillary Thinks African Americans are Super Predators.”

If similar ads had appeared on TV, radio or in newspapers, journalists and advocacy groups would have fact-checked them. Negative ads in those media are closely monitored because historically they have influenced elections — most notably in 1988, when a television ad accused presidential candidate Michael Dukakis of “weak-on-crime” policies that enabled a furloughed prisoner named Willie Horton to commit rape.

The Trump ads may have been effective as well. But since they supposedly appeared on Facebook, nobody can say for sure if they ran, what they said or whom they targeted. Even though it’s the world’s largest social network, what happens on Facebook stays on Facebook.

The nature of online advertising is such that ads appear on people’s screens for just a few hours, and are limited to the audience that the advertiser has chosen. So, for example, if an advertiser micro-targets a group such as 40-year-old female motorcyclists in Nashville, Tennessee, (Facebook audience estimate: 1,300 people) with a misleading ad, it’s unlikely anyone other than the bikers will ever see those ads. Yesterday, 10 months after Trump was elected, Facebook officials acknowledged discovering that a Russian “troll farm” paid $100,000 during the campaign to place political ads on issues such as gun rights and immigration, The Washington Post reported.

With online ads, “you can go as narrow as you want, as false as you want and there is no accountability,” said Craig Aaron, president and CEO of Free Press, a public interest media and technology advocacy group.

ProPublica wants to change that. Today we are launching a crowdsourcing tool that will gather political ads from Facebook, the biggest online platform for political discourse. We’re calling it the Political Ad Collector — or PAC, in a nod to the Political Action Committees that fund many of today’s political ads.

Download the Extension You can download the Facebook Political Ad Collector from the Chrome Web Store or the Mozilla Add-ons Store.

We will begin using the PAC this month to track ads during the run-up to the German parliamentary election, which will be held on Sept. 24. The election has drawn international attention as a referendum on Chancellor Angela Merkel’s refugee policies, and a test of the strength of an anti-immigration party, Alternative for Germany (AfD).

We plan to monitor other elections, including the midterm elections in the U.S. In the U.S., information about politicians’ use of online ads is especially sparse because of loopholes in the campaign finance laws that allow candidates to report fewer details about their online advertising than about other types of advertising.

We are working with three news outlets in Germany — Spiegel Online, Süddeutsche Zeitung and Tagesschau. They will ask their readers to install our tool, and will use it themselves to monitor ads during the election.

The tool is a small piece of software that users can add to their web browser (Chrome). When users log into Facebook, the tool will collect the ads displayed on the user’s news feed and guess which ones are political based on an algorithm built by ProPublica.

One benefit for interested users is that the tool will show them Facebook political ads that weren’t aimed at their demographic group, and that they wouldn’t ordinarily see.

Facebook’s Secret Censorship Rules Protect White Men From Hate Speech But Not Black Children A trove of internal documents sheds light on the algorithms that Facebook’s censors use to differentiate between hate speech and legitimate political expression.

The tool does not collect any personally identifiable information, and we will not know which ads are shown to which user. The political ads that we collect will be contributed to a public database that will allow the public to see them all.

Facebook gives users more information about why a particular ad is targeted to them than other online platforms provide to their customers. Our tool will also collect that targeting information provided by Facebook, which may help illuminate what viewership the ads are trying to reach.

After the U.S. presidential election, Facebook launched its own transparency efforts. Facebook CEO Mark Zuckerberg has rolled out a series of initiatives to tackle fake news on its site. And although it doesn’t fact-check ads, Facebook does require advertisers to comply with the law, which includes prohibitions against deceptive advertising. This week Facebook said it had shut down the “inauthentic accounts” affiliated with Russia that had placed ads during the 2016 election cycle and is taking steps to prevent similar accounts from popping up in the future.

Still, more can be done to hold politicians, PACs and others accountable for the messages they spread online. We hope that by monitoring political advertising on Facebook, we can increase the transparency and accountability of elections around the world.

Please join us!