Alexander Malkevich, head of the Moscow-based Foundation for National Values Protection, which monitors cases of Russian citizens who have been detained or arrested abroad, called it “absolutely unacceptable” for Bogacheva to be held.

It has been assumed from the beginning that none of the Russians indicted by Mueller would ever see the inside of an American courtroom, as long as they kept clear of the United States.

Bogacheva’s detention raised, however briefly, the possibility of a criminal proceeding against one of the employees of the organization that was at the center of alleged Russian social media interference in the American election.

Known as the “troll factory,” the Internet Research Agency in St. Petersburg was linked to Yevgeniy Prigozhin, called “Putin’s chef” because he used to provide catering for Russian President Vladmir Putin.

Maria Butina, a Russian who is currently in prison after pleading guilty to working as an unregistered foreign agent, was not prosecuted by Mueller’s team, but by the U.S. attorney for the District of Columbia.

Bogacheva went by the pseudonym Anna Trigga when she was associated with a violent right-wing Russian nationalist organization, according to an investigation by Radio Liberty. She later went to work for the Internet Research Agency, and made a 2014 trip to the United States to gather information, some Russian accounts suggested, as a way to stay out of trouble with the Russian secret services.

The first reports of her detention were made public Tuesday morning. The fact that Belarus, which has a complicated but close relationship with Russia, would act on an American warrant was surprising. Russian media pounced on the story, as did Malkevich’s organization and the Russian embassy in Minsk.

“This is right,” Malkevich said, “because Russian citizens should feel they are protected anywhere in the world from total arbitrariness.”

Belarus relented, he suggested, under the public scrutiny.

Though the embassy was “very active,” he said, he dismissed the notion that Moscow had brought official pressure to bear. But it was clear that Russia would do everything within its power to prevent her or any other defendant in the election-interference investigation from being extradited to face an American trial.

Malkevich said he suspects that the decision to detain Bogacheva, on an Interpol “red notice,” was made by a low-level official.

Nonetheless, the Belarusian government of President Alexander Lukashenko has been wary of too-close a relationship with Russia, and from time to time has made overtures to the West. But from the Kremlin’s point of view, this would be a very high-stakes case.

Belarusian authorities did not comment, except for a statement by the prosecutor general’s office saying there were no grounds to hold Bogacheva on the basis of the American charges.

According to the February 2018 indictment, Bogacheva worked for the Internet Research Agency from at least April to July 2014 overseeing its data analysis project, and was one of the co-defendants who allegedly traveled to the United States “under false pretenses for the purpose of collecting intelligence.”

She allegedly planned travel itineraries, purchased cameras, drop phones and other equipment for defendants who made U.S. visits, and discussed “evacuation scenarios” and other security measures, according to the indictment. She and three others who applied to the State Department for visas allegedly claimed they were traveling for personal reasons and did not fully disclose their place of employment to hide their IRA connections, the indictment said.

Prosecutors said only Bogacheva and co-defendant Aleksandra Krylova, allegedly the plot’s third-highest ranking member, received visas. Between June 4 and June 26, 2014, the pair crossed the United States, stopping in Nevada, California, New Mexico, Colorado, Illinois, Michigan, Louisiana, Texas, and New York to gather information for the plot to interfere in the U.S. political system, the indictment alleged.

Spencer Hsu in Washington contributed to this report.