One day before he was to testify about an alleged cover-up after a deadly terrorist bombing at a Jewish center in Argentina, a federal prosecutor was found dead of a gunshot wound in his Buenos Aires apartment.

Alberto Nisman's body was found Sunday. Officials say they also found a gun, but no note that might indicate his death was a suicide, according to local daily Clarin. An autopsy is being performed today, the newspaper adds.

Update at 5 p.m. ET: No Sign Of Outside Involvement In Nisman's Death

The prosecutor investigating Nisman's death says he died from a single shot to the head from a .22 caliber handgun found at the scene and that there's no evidence that anyone other than the prosecutor was involved in his death.

But investigators are still regarding the death as suspicious. They're trying to find out who owns the gun (Nisman didn't, they say) and are awaiting results from more forensic tests, Clarin reports.

Citing orders from Argentina's president, the country's intelligence agency says it "will declassify data relating to intelligence personnel implicated in the wiretaps related to the 1994 AMIA bombing investigation," according to the Buenos Aires Herald.

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Nisman's death comes days after he accused President Cristina Fernandez de Kirchner of being complicit in a cover-up of a deadly attack that has been a source of controversy since it occurred 20 years ago.

NPR's Lourdes Garcia-Navarro reports:

"It was the worst terror attack in Argentine history — 85 people were killed, and more than 200 injured, when the Argentine-Israeli Mutual Association was bombed in 1994."Alberto Nisman was the lead investigator on the case. No one has been convicted for the attack, but prosecutors have accused Iran and Hezbollah over the bombing."Nisman was supposed to testify Monday, presenting recordings from phone taps in a closed-door hearing. This past week, he accused Fernandez and other senior Argentine officials of agreeing not to punish at least two former Iranians in the case in order to further Argentina's 'commercial, political and geopolitical interests.'"In particular, he alleged that Argentina was trying to cement a deal swapping wheat for oil in the energy-starved country."

Nisman died four days after he "filed a criminal complaint against President Cristina Kirchner, her foreign minister and others," The Wall Street Journal reported last week, noting that the prosecutor had also sought to freeze some $23 million in assets.

Nisman had been investigating the AMIA bombing for the past 10 years, after being appointed by Kirchner's late husband, former president Nestor Kirchner.

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