BUENOS AIRES — A federal judge in Argentina on Tuesday indicted a computer expert on charges that he acted as an accomplice in the January 2015 death of the prosecutor Alberto Nisman. It is the first time a judge has formally called the politically fraught case, which remains unsolved, a homicide.

In the indictment, Judge Julián Ercolini contended that the expert, Diego Lagomarsino, lent Mr. Nisman, his boss, the gun that was used to kill him as part of a conspiracy to create the impression that the prosecutor had committed suicide.

But the court filing does not shed significant new light on the mysterious death of Mr. Nisman, who was leading an investigation into the 1994 bombing of a Jewish community center in Buenos Aires that killed 85 people.

Shortly before he died, Mr. Nisman accused Cristina Fernández de Kirchner, then the president, and other top officials of covering up Iran’s possible involvement in the bombing, the deadliest terrorist attack in Argentina’s history.