A forthcoming peer-reviewed study of paid political ads that appeared on Facebook in the weeks just prior to the 2016 presidential election shows that of 228 groups purchasing ads on hot-button issues, 122 — more than half — were submitted by “suspicious” groups whose identities may never be known.

The University of Wisconsin researchers, led by Professor Young Mie Kim, defined “suspicious” as meaning there was no publicly available information on who was behind the groups.

The researchers looked at Federal Election Commission, IRS, and other public databases and could find no information about the groups. Campaign finance laws compel less disclosure for online ads than for print, television, or radio ads.

One out of six of the suspicious groups later turned out to be Russian groups, according to the study, as identified by investigations into Russian interference in the U.S. elections.

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The study tracked 5 million paid ads on Facebook that reached almost 10,000 people in the Sept. 28 through Nov. 8 time period of 2016.

The volume of ads sponsored by these non-FEC identified groups was four times larger than those from FEC-identified groups. They also were targeted geographically, by demography and by issue.

“Divisive issue campaigns clearly targeted battleground states, including Pennsylvania and Wisconsin, where traditional Democratic strongholds supported Trump by a razor thin margin,” the researchers noted.

So, for example, lower income households were targeted more with ads about immigration and racial conflict. Middle income households were targeted more with nationalistic appeals. Whites were the main racial category targeted across income groups.

Other hot button issues in the ads were abortion, gay rights, fears of terrorism, and scandals of the individual candidates.

“To the best of our knowledge,” the researchers wrote, “this is the first, large-scale, systematic empirical analysis that investigates who operated divisive issue campaigns on Facebook . . . and who was targeted by these issue campaigns.”

The study will be published in a forthcoming issue of the scholarly journal Political Communication.

Issue One, a bipartisan group that lobbies to change campaign finance laws and is chaired by former New Jersey Sen. Bill Bradley, a Democrat, and Wyoming Sen. Alan Simpson, a Republican, touted the study as evidence supporting the so-called “honest ads” bill sponsored by Sen. Amy Klobuchar, D-Minn, and co-sponsored by Sen. John McCain, the lone Republican, and 20 other Senate Democrats.

Facebook and Twitter have endorsed the bill.