By contrast, allies of Mrs. Kirchner, who has denied any wrongdoing, characterized the new forensic report as an effort by the current government to further undermine her image. Mrs. Kirchner faces charges in several corruption investigations.

But in the run-up to the election, Mrs. Kirchner has a mystery of her own to point to: The disappearance of Santiago Maldonado, an indigenous rights activist who, supporters say, vanished after border guards took him into custody.

The disappearance has outraged many Argentines, and Mrs. Kirchner contends that the government is simply putting forward the new allegations about Mr. Nisman’s death in order to distract attention from the case now unfolding on its watch.

“This is an immense smoke bomb to hide Santiago Maldonado,” Mrs. Kirchner said in a radio interview. Mr. Maldonado’s family and human rights groups have called for a protest on Sunday to mark the two-month anniversary of his disappearance.

The latest forensic investigation into Mr. Nisman’s death was carried out by a team of 28 experts. Over the course of nine months, they reconstructed the scene where his body was found in his bathroom, with a single gunshot wound to the head. They concluded the prosecutor was killed by two people, according to the senior judicial official, who has seen the report.

The forensic experts discovered several injuries on Mr. Nisman’s body — including a nasal fracture, a hematoma in his kidney, lesions on his legs and a wound on the palm of his hand — that they say are consistent with an attack on the prosecutor before he was killed. According to the official, investigators also said they found ketamine, an anesthetic, in Mr. Nisman’s blood, which they suspect was used to sedate the prosecutor before he was shot. No gunpowder residue was found on his hands, which they said made the suicide theory implausible.