In an unusual move against Russian-style election deception inside the United States, Facebook said on Saturday it had removed five accounts of Americans who used its platform improperly in the hard-fought Alabama Senate election last year.

Among those whose accounts were removed was a prominent social media researcher who worked on the Alabama operation, Jonathon Morgan, according to a person briefed on the company’s action. Facebook did not name those whose accounts were closed, and it was not immediately possible to identify the others.

The company acted in response to reports in The New York Times, The Washington Post and other news outlets in recent days that a small group of social media experts had secretly used deceptive tactics in the Alabama race that were explicitly modeled on Russia’s meddling in the 2016 American presidential election. The group sought to split the conservative vote to undermine the Republican candidate, Roy S. Moore, and to boost support for the Democrat, Doug Jones, who won by a small margin.

Mr. Morgan has said the operation was an experiment to learn more about the kinds of methods Russia had used, and not to influence the outcome of the race. But the episode has sparked fears that the fraudulent Russian operations on social media could be imitated widely by American political operatives and further undermine voters’ ability to sort truth from fakery as they choose candidates.