FILE PHOTO: Silhouettes of mobile users are seen next to a screen projection of Facebook logo in this picture illustration taken March 28, 2018. REUTERS/Dado Ruvic/Illustration/File photo

(Reuters) - Most of the political ads about divisive issues that ran on Facebook Inc FB.O before the 2016 U.S. presidential election were sponsored by "suspicious groups" with no publicly available information about them, according to a study released on Monday and based on a database of five million ads on Facebook.

One in six of those groups was linked to Russia, according to a University of Wisconsin-Madison study here, and the identities of the rest of the 122 groups that are labeled "suspicious" are still unknown, an indication of the influence of "astroturf" or shell companies in U.S. politics.

Over a quarter of the suspicious ads mentioned Donald Trump or Hillary Clinton, two of the presidential candidates in the election, and 9 percent expressly advocated for or against individual candidates.

Most other ads deliberately avoided mentioning candidate names while still getting the message out by doing things like supporting policies pushed by candidates, Young Mie Kim, the lead researcher said.

The researchers labeled suspicious ad-buyers as groups with pages that have been inactive, inaccessible, removed or banned by Facebook since the election and there was no information available publicly about them.

Project DATA, the research team, also found that voters were also disproportionately targeted in swing states like Wisconsin and Pennsylvania with ads that focused on issues like guns, immigration and race relations.

Facebook Chief Executive Officer Mark Zuckerberg has announced a crackdown on who buys ads about divisive issues, saying this month that the company would require every such advertiser to confirm their identity and location.