It is doubtful that any of the named Iranians will ever appear in an American courtroom. In that respect, the indictment is similar to one the Justice Department issued two years ago against members of Unit 61398 of the People’s Liberation Army of China, which it accused of stealing data from American corporations. The Chinese have never been arrested.

But the administration argues that such indictments send a strong signal and make it difficult for those who are indicted to travel, for fear of extradition.

On Tuesday, the Justice Department indicted two other hackers who it said were members of the Syrian Electronic Army, which has supported the government of Bashar al-Assad, and it believes that it has a chance to gain custody of one of them. On Wednesday, the department obtained a guilty plea from a Chinese national living in Canada, Su Bin, whom it accused of mounting a cybercampaign to steal the designs of military aircraft from Boeing, on behalf of Chinese intelligence agents.

The Iran indictment comes eight months after the nuclear deal reached between Tehran and six other nations, including the United States, which appeared to be putting Tehran and Washington on a track toward a more productive relationship after 35 years of enmity. But Iranian missile launches in recent months — also organized by the Guards — have led to calls in Congress for new sanctions.

The indictment appeared to be part of an American effort to keep Iran from shifting activity from its nuclear program to its growing corps of cyberwarriors, some of whom work directly for the government, while others, like those named in the indictment, seem to be contractors.

As a measure of the importance the administration placed on the indictment, it was announced by Attorney General Loretta E. Lynch, in a news conference in Washington with Preet Bharara, the United States attorney for the Southern District of New York, where the indictment was handed up. It was unclear how long it had been under seal.