Facebook says it has removed 32 accounts and pages believed to have been set up to influence the mid-term US elections in November.
It said it was in the "very early" stages of the investigation and did not yet know who was behind the pages.
It said the creators had gone to greater lengths to hide their identities than a Russia-based campaign to disrupt the 2016 presidential vote.
It described attempts to erase election interference as an "arms race."
What did Facebook discover?
The social network said in a blog that it had identified 17 suspect profiles on Facebook and seven Instagram accounts.
It said that there were more than 9,500 Facebook posts created by the accounts and one piece of content on Instagram.