U.S. Rep. Ileana Ros-Lehtinen (R-FL), chairwoman of the Middle East and North Africa Subcommittee, and Sen. Marco Rubio (R-FL) are calling for an independent investigation into the mysterious death of a special prosecutor in Argentina who died of a bullet wound the day before he was set to testify before a Congressional hearing.

Alberto Nisman, found dead Sunday in his Buenos Aires apartment, was shot once with a .22 caliber handgun on the right temple, according to findings of a preliminary autopsy released Monday. Prosecutors said there was no indication that anybody else was involved in the death.

Nisman, 51, had accused Argentina President Cristina Fernandez of shielding Iranian suspects in the country's worst terrorist attack — the 1994 bombing of the Argentinian-Israeli Mutual Association in Buenos Aires, which killed 85 people and injured more than 200. He was set to testify about the bombings on Monday.

Nisman had 10 federal police assigned to his protection, according to a statement from the Security Ministry, which did not provide details on where the officers were posted when Nisman died.

“My office and I were in frequent communication with Alberto to discuss the AMIA case and the threat that Iran and its proxy Hezbollah poses to hemispheric security," said a statement released by Ros-Lehtinen. "Under these mysterious circumstances, I urge an independent, transparent, and neutral entity to fully investigate the facts surrounding his death.”

In a separate statement, Sen. Marco Rubio said the news of  Nisman’s death is "very concerning on many levels."

"I intend to explore ways to ensure Dr. Nisman’s thorough investigation into Iran’s role in the AMIA and his lengthy reports on Iran’s penetration into Latin America receive the attention they deserve," he said.

“I call on Argentinian authorities to ensure that a credible and transparent investigation into his death is carried out. I also urge the Fernandez government to resist any efforts to whitewash Iran’s true role in the 1994 bombing," he added.

In an emergency ruling, judge Ariel Lijo said the evidence on which Nisman based his allegations – including 300 CDs with recorded conversations – is to be kept sealed in his office, La Nación newspaper reported.

Government officials, including Cabinet chief Jorge Capitanich and President Fernandez, have declined to comment on the investigation.

Argentina has one of the largest concentrations of Jews outside of Israel, with estimates ranging around 200,000, mostly in Buenos Aires.

Israel's foreign ministry expressed "deep sorrow" over Nisman's death.

"Nisman, a courageous, venerable jurist who fought intrepidly for justice, acted with determination to expose the identities of the terrorists and their dispatchers," a ministry statement said.

Last week, Nisman accused Fernandez and other senior Argentine officials of agreeing not to punish at least two former Iranian officials in the case. He asked a judge to call Fernandez and others, including Foreign Minister Hector Timerman, for questioning.

"The president and her foreign minister took the criminal decision to fabricate Iran's innocence to sate Argentina's commercial, political and geopolitical interests," Nisman said last week.

After years of inconclusive investigations and failed trials, an Argentine judge in 2006 accepted Nisman's request to order the arrest of a former Iranian president, foreign minister and other officials. Interpol later put most of them on its most-wanted list.

But Argentina and Iran reached an agreement in 2013 to jointly investigate the attack, a move viewed with skepticism by Jewish leaders who feared it would undermine Nisman's probe.

Within hours after news of Nisman's death spread, a well-known group called "Indignant Argentines" called for demonstrations later Monday in several areas of Buenos Aires.

"Nisman died but his denouncement does not," Sergio Bergman, a prominent rabbi in Buenos Aires, posted on Twitter. "Our sorrow and condemnation will result in more memory, truth and justice!

The Associated Press contributed to this report.

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