No evidence has been found that anyone else was involved in the death of prosecutor Alberto Nisman, who had criticised Argentina's president over a 1994 terror attack, officials say.
An autopsy found "no intervention" in his death, they said.
Mr Nisman had been investigating the 1994 bombing of a Jewish centre in Buenos Aires in which 85 people died.
He had accused President Cristina Fernandez de Kirchner of involvement in a plot to cover up Iran's alleged role.
The government has dismissed the allegations as ridiculous.
Mr Nisman, 51, was found shot dead in the bathroom of his home, with a gun lying beside him. The office of investigating prosecutor Viviana Fein said that she was still awaiting the results of other tests "in order to be able to rule out any other hypothesis".
'No suicide note'
Speaking earlier, Ms Fein said he had been shot once.
She ruled out burglary as a motive and also said that no suicide note had been found in the flat.
Mr Nisman had been due to give evidence at a congressional committee hearing on Monday to outline his accusations against President Fernandez and other officials.
Congresswoman Cornelia Schmidt-Liermann told AP news agency she had been due to collect Mr Nisman from his home and accompany him when he went to give his testimony.
"Everybody who had contact with him the last 24 hours says he was confident," she said.
"There is no indication, under any circumstances, that he killed himself."
Last Wednesday, Mr Nisman published a 300-page report alleging that the president and Foreign Minister Hector Timerman had opened a secret back-channel to Iranians suspected of involvement in the bombing of the community centre.
Mr Nisman alleged that the scheme was intended to clear the Iranian suspects in order to facilitate a trade deal between Iran and Argentina.
The security ministry said in a statement that Mr Nisman's bodyguards had raised the alarm after he failed to answer their phone calls on Sunday.
They fetched Mr Nisman's mother and tried to enter his apartment, but the door was locked from the inside with a key still in the lock.
After a locksmith gained access, they found Mr Nisman's body in the bathroom.
According to the statement, a gun and a cartridge shell were found next to his body.