TJ Smith fixes an 'I Voted' sticker on his shirt after casting his ballot at a polling station during early voting for the mid-term elections in the Lakeview Terrace neighborhood of Los Angeles, California on November 4, 2018.

Twitter bots, including thousands originating in Russia and Iran, as well as those created in the United States, were a much more active presence than previously known during last year's midterm elections.

More than 15,000 Twitter accounts identified as robots used the #ivoted hashtag on Election Day, according to a new study, which came from the University of Southern California's Information Sciences Institute and a Swiss researcher. In virtually every U.S. state, more than a fifth of those posting about the elections on Twitter in the weeks before Election Day were robots.

In contrast, Twitter released findings last week that focused mainly on foreign efforts. Days before the midterms, Twitter made a splash when it confirmed it had removed 10,000 bots pretending to be Democrats.

Between Oct. 6 and Nov. 19, nearly two weeks after Election Day, the researchers behind Monday's report identified more than 200,000 bots posting about the midterm elections, compared to about 750,000 humans.

The new figures are included in a report published by researchers who collected and analyzed millions of Twitter posts, and in a subsequent study to be released in the coming days.

The research provides more detail about interference operations that Twitter said on Thursday were found to originate within Iran, Venezuela and Russia.

Thousands of those bots could be traced to Russia and several hundred could be traced to Iran, according to Emilio Ferrara, one of the study's four authors and the principal investigator in the Machine Intelligence and Data Science group at USC's Information Sciences Institute.

Still, much of the bot activity appeared to originate in the United States or had no clear provenance, he said. External researchers do not have as much information as Twitter and law enforcement about where accounts originate.

In a statement for this story, Twitter spokesman Ian Plunkett called the company's report the "the single source of truth from our side on the election." The Department of Justice declined to comment, and the Department of Homeland Security did not respond to a request for comment.

In August, Twitter, Facebook and Alphabet, Google's parent company, removed hundreds of accounts tied to Iran ahead of the midterm races. Two reports prepared for the U.S. Senate and released in December found that Russia used every major social media platform to interfere in the 2016 presidential election.

Disinformation will be in the spotlight in the coming months as Twitter and other platforms seek to avoid a repeat of 2016, in which the Russian government amplified American cultural divides using deceptive social media practices. The companies are working to prevent fake actors from using their services while protecting the ability of their human users to express political viewpoints.