Judge Ercolini accused the bodyguards of negligence for allowing the weapon to enter Mr. Nisman’s apartment. He also faulted them for not detecting the shooting when it happened, which the judge said allowed the culprit or culprits to leave the scene undetected and delayed the discovery of the body.
He ordered Mr. Lagomarsino to remain under electronic monitoring pending trial and froze his assets and bank accounts.
Mr. Lagomarsino acknowledged shortly after the prosecutor’s death that the bullet that killed Mr. Nisman, who was shot in the head, came from a handgun he owned. But he has consistently said that Mr. Nisman asked to borrow the weapon after receiving threats against him and his daughters.
Mr. Lagomarsino, who learned of the indictment while he was being interviewed on live television, insisted that he was innocent.
“Alberto Nisman ruined my life,” Mr. Lagomarsino said in a radio interview shortly before Judge Ercolini’s indictment was made public. “He had no idea he would be putting me in the mess that he did.”
Mr. Lagomarsino said the courts should judge him for lending Mr. Nisman a gun “if I committed a crime,” but he insisted that “I have nothing to do with the rest.”
In the 656-page indictment, Judge Ercolini noted that “Nisman was killed with Diego Lagomarsino’s weapon and, at the same time, he was the last person who entered the prosecutor’s apartment.”
Judge Ercolini added that in the wake of Mr. Nisman’s death there were numerous events that led officials to “publicly push the idea of suicide,” contributing to an “almost unambiguous public certainty that Nisman had taken his own life.”
Judge Ercolini acknowledged that while he was accusing Mr. Lagomarsino of involvement in the homicide, the actual perpetrators “remain unknown.”
In his accusation, Judge Ercolini cast blame on officials from the government of Mrs. Kirchner, who, along with friendly media outlets, promoted the theory that Mr. Nisman committed suicide.
“The death of prosecutor Nisman was not a suicide and was produced by third parties in an intentional manner,” Judge Ercolini asserted.
Mr. Nisman was found dead in his bathroom with a gunshot wound to the head hours before he was scheduled to testify at a congressional hearing about his accusation that Mrs. Kirchner and several members of her administration conspired with Iran to cover up Iran’s possible involvement in the bombing of the Jewish community center.
Judge Ercolini’s indictment follows the release in September of a report by forensic experts who concluded that Mr. Nisman had been murdered. That finding contradicted the assessment of another team of experts who determined that Mr. Nisman had most likely killed himself.
Mr. Nisman’s accusation against Mrs. Kirchner and other top officials in her administration has received renewed attention since her party lost the presidency in 2015. President Mauricio Macri has said he believes that Mr. Nisman was murdered.
A separate criminal investigation into the bombing is continuing.
Mrs. Kirchner, who was recently sworn in as a senator, was charged with treason this month in connection with the bombing case. An appeals court recently ruled that the treason charge was not warranted, but it allowed the indictment against her and other officials to proceed.
As a senator, Mrs. Kirchner enjoys immunity from detention, which can be stripped only with the approval of two-thirds of the senators present at the time of the vote.