The Justice Department said on Wednesday that it had charged four Russian soldiers with torturing an American living in the war-ravaged region of Kherson in Ukraine, using a war crimes statute for the first time since it was enacted nearly three decades ago.

The indictment, unsealed in Virginia, could be followed by other charges against Russians found to have committed “atrocities on the largest scale in any European armed conflict since the Second World War,” Attorney General Merrick B. Garland said in announcing the prosecution.

For the first time, Mr. Garland also acknowledged that the department had begun a formal investigation of the “murder of more than 30 Americans” by Hamas fighters during the Oct. 7 attack in Israel, either under the same war crimes law being used against the Russian soldiers or through the use of antiterrorism statutes. He provided no other details.

All four of those charged now live in Russia. The prospects that they would travel abroad anytime soon, where they could be captured, are remote, officials who joined Mr. Garland at the news conference said.

But the prosecutions are part of a broader effort by the Justice Department, the F.B.I. and the Department of Homeland Security to hold Russian military officials and proxy forces accountable for brutal acts committed against the relative handful of Americans living, fighting or working in Ukraine.

The American in the Ukraine case was not identified in court filings. But prosecutors said he was abducted in April 2022 from his home in Mylove, a village in southern Ukraine, despite telling Russian forces moving into the area that he was not a combatant and had been living in the country with his wife since 2021.

During the victim’s roughly 10 days in captivity, soldiers with the Russian Armed Forces and paramilitaries with the so-called Donetsk People’s Republic beat him brutally with their fists and the butts of their guns and threatened to sexually assault him. In one harrowing episode, according to prosecutors, they hauled him from the building where he was being held captive to stage a mock execution — which ended when a bullet was fired inches from his temple as he knelt on the ground.

The two commanders, identified as Suren Seiranovich Mkrtchyan and Dmitry Budnik, along with two subordinates identified only by their first names, subjected the man to two particularly vicious interrogation sessions, prosecutors said. They stripped him naked, then photographed him, all the while accusing him of being an American government operative.

They repeatedly taunted him with death threats, telling him “good night” through an interpreter and warning that he was about to be put “to sleep,” Mr. Garland said.

The American was eventually released and evacuated back to the United States, officials added.

Christopher A. Wray, the F.B.I. director, who joined Mr. Garland for the announcement, said the prosecutions were a significant step. “We’re resolved to hold war criminals accountable no matter where they are or how long it takes,” he said.

Few Americans have been killed or injured in the war. But Justice Department officials began collecting information on potential American victims from the first hours of the Russian invasion.

In March, Christian Levesque, a veteran war crimes prosecutor in the department’s human rights section, told The New York Times that her team was examining “anything at all” — from news reports to intelligence — that could yield evidence of wrongdoing at the hands of Russia.

To help coordinate the overall effort to investigate war crimes, Mr. Garland appointed Eli Rosenbaum, a veteran prosecutor, last June. Mr. Rosenbaum is best known for his dogged pursuit of Nazi war criminals and the unmasking in the 1980s of the role that the former secretary general of the United Nations, Kurt Waldheim, played in the mass killings of civilians during World War II.

The department’s initial efforts to investigate potential war crimes were hampered by the dangerous situation during the early stages of the war, when the department’s presence was limited to a single official, working out of the embassy in Kyiv. But the number of U.S. investigators, including F.B.I. agents, has steadily increased, and American investigators have worked closely with Ukraine’s national police and other law enforcement agencies.

The Justice Department has also embraced a supporting role, providing Ukraine’s overburdened prosecutors and police with logistical help, training and direct assistance in bringing charges of war crimes by Russians in Ukraine’s courts.

The department’s work is separate from investigations by the International Criminal Court, which issued a warrant for the arrest of President Vladimir V. Putin of Russia last March, saying he bore criminal responsibility for the abduction and deportation of Ukrainian children.

The United States has never joined the International Criminal Court out of concern that it could try to prosecute Americans.