Sara Garfunkel, mother of late prosecutor Alberto Nisman, leaves the office of prosecutor Viviana Fein, who is investigating Nisman's death, in Buenos Aires January 22, 2015. REUTERS/Marcos Brindicci

By Hugh Bronstein

BUENOS AIRES (Reuters) - Argentina suspects rogue agents from its own intelligence services were behind the death of a state prosecutor investigating the deadly 1994 bombing of a Jewish community centre in Buenos Aires.

Alberto Nisman was found dead in his apartment late on Sunday, a gunshot wound to his head and a 22 caliber pistol by his side along with a single shell casing.

He had been scheduled to appear before Congress on Monday to answer questions about his allegation that President Cristina Fernandez conspired to derail his investigation of the attack.

His death and a blizzard of conspiracy theories around it have rocked Argentina.

The government says Nisman's allegations and his death were linked to a power struggle at Argentina's intelligence agency and agents who had recently been fired.

It says they deliberately misled Nisman and may have had a hand in writing parts of his 350-page complaint.

"When he was alive they needed him to present the charges against the president. Then, undoubtedly, it was useful to have him dead," the president's chief of staff, Anibal Fernandez, said on Friday.

Argentine courts have accused a group of Iranians of planting the 1994 bomb, which killed 85 people.

Nisman claimed last week that President Fernandez opened a secret back channel to Iran to cover up Tehran's alleged involvement in the bombing and gain access to Iranian oil needed to help close Argentina's $7 billion per year energy deficit.

Fernandez's government called the accusation absurd.

Iran has repeatedly denied any link to the bomb attack.

The Argentine government's chief of staff said on Friday that he didn't believe Nisman even wrote his own report.

"I have worked quite a bit with prosecutor Nisman. I know he was a well qualified expert in the law. He could not have written this nonsense," he said. "It is totally clear he had nothing to do with it, but there were people around him who had a different agenda."

Although the government says there was a conspiracy to falsely accuse the president and then do away with Nisman, no one has been arrested in the case so far.

On Friday afternoon, the lead investigator in the case said the man who lent Nisman the gun that was by his side when he was found dead was now banned from leaving the country. The man gave the gun to Nisman the day before his death.

Diego Lagomarsino had turned himself in for questioning after the news broke but has not been taken into custody.

Officials initially said Nisman's death looked like suicide, but the lead investigator says it is suspicious and that all leads are being followed.

The head of Argentine intelligence was replaced in December, resulting in the firing of agents who had been helping with Nisman's investigation. Nisman had accused agents from another faction within the state intelligence apparatus of being part of Fernandez's alleged plot to clear the Iranian suspects.

One of those fired in the December shakeup was Antonio Stiusso, a senior spy who had helped Nisman with the probe.

The government says Stiusso falsely told Nisman that two men implicated in the case against the president were state intelligence agents. Wire tapped phone conversations of the two were key to Nisman's accusation that the government was trying to whitewash the 1994 car bombing.

President Fernandez said on Thursday that she did not believe Nisman took his own life.

"Nisman's accusation never was, in itself, the true operation against the government ... The true operation against the government was his death, after accusing the government," she said in a post on Facebook.

Those close to the late prosecutor have doubted from the beginning that he killed himself. Friends described him as upbeat ahead of his scheduled appearance before Congress on Monday.

(Additional reporting by Sarah Marsh, Nicolas Misculin and Walter Bianchi; Editing by Kieran Murray and Andrew Hay)