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Rodrigo Abd, Associated Press
People gather outside the AMIA Jewish community center asking for "Justice" in the death of a prosecutor who had accused Argentina’s president of a criminal conspiracy, in Buenos Aires, Argentina, Wednesday, Jan. 21, 2015. Special prosecutor Alberto Nisman, who had been investigating the 1994 bombing of the Jewish community center that killed 85 people and who accused President Cristina Fernandez of shielding Iranian suspects, was found dead from a gunshot to the head, in his apartment late Sunday, hours before he was to testify in a Congressional hearing about the case.

BUENOS AIRES, Argentina — Argentine President Cristina Fernandez said Thursday she's "convinced" prosecutor Alberto Nisman did not commit suicide as more questions arose in the death of the man who had accused the president of a cover-up in the nation's worst terrorist attack.

In a letter published by the state news agency Telam, Fernandez said all the questions about Nisman's death "have been converted into certainty. The suicide (I'm convinced) was not a suicide."

Fernandez' letter contrasts with the one she wrote Monday saying she believed Nisman took his life.

The 51-year-old Nisman was found slumped in the bathroom of his apartment Sunday night with a bullet wound in his head. He was lying next to a .22-caliber handgun and a bullet casing.

Four days, before Nisman gave a judge a 289-page report alleging Fernandez secretly reached a deal to prevent prosecution of former Iranian officials accused of involvement in the 1994 car bombing of Argentina's largest Jewish center.

Fernandez dismissed those accusations in Thursday's letter.

On Wednesday, a locksmith said the service door wasn't fully locked at the apartment where Nisman was found shot dead. Investigators have also revealed the existence of a previously unknown entry as new doubts about what happened continued to pop up.

Viviana Fein, the lead investigator into Nisman's death, said Monday the death appeared to be suicide and there were no indications anyone else was involved. The apartment's door was locked from the inside and there were no signs it had been forced, she said.

But family and friends of Nisman immediately rejected the finding and protesters took to the streets demanding justice for the prosecutor who had spent 10 years investigating the 1994 bombing of a Jewish community center in Buenos Aires that killed 85 people.

Details began trickling out that raise questions about the suicide hypothesis.

No suicide note was found and a test of Nisman's hand showed no gunpowder residue, though Fein said that may have been due to the small caliber of the gun. Also feeding suspicion was the rapid appearance of national Security Secretary Sergio Berni at the apartment, since he is a government, not a judicial, official. He denied he altered the crime scene.

Then the locksmith who opened the back door to give investigators access to Nisman's apartment said it hadn't been properly locked, raising speculation about whether a killer might have entered or exited the 13th-story apartment.

After testifying to investigators Tuesday, the locksmith, who gave his name to journalists only as Walter, said he was called to let authorities into Nisman's apartment.

The front door had a keyless system so he decided to go in through the service door. Authorities said Monday that Nisman's mother hadn't been able to open the service door because a key was in the lock on the other side.

"The service door wasn't closed. I simply pushed the key and entered in two minutes," the locksmith told reporters.

He said he was able to quickly open the door with the help of a hook. "It took me longer to pack up my things (tools) than to open the door."

He added, "If someone entered or not, I don't know."

The official news agency Telam, meanwhile, said investigators had found a third access to the home, a narrow passage holding air conditioning equipment that connects to a neighboring apartment occupied by an unidentified foreigner. They were investigating a seemingly recent footprint and fingerprint found inside.

Fein said the gun found beside Nisman was registered to another man, Diego Lagomarsino, described by officials as a colleague of Nisman, who had given it to him.

The death, and Tuesday's release of Nisman's full report, caused a crisis for the government, which scrambled to promise "maximum transparency and cooperation" in the investigation.

The report accused Fernandez and Foreign Minister Hector Timerman of reaching agreement with Iran to avoid prosecution of eight Iranians, including former senior officials, charged with involvement in the bombing. He said that would open a lucrative trade in Argentine grains and meat for Iranian oil.

In the end, Interpol never dropped its "red notices" for the arrest of five of the Iranians, and the government said trade with Iran has diminished in recent years.

Writing with passion, Nisman called it "a criminal plan to erase at a stroke the serious accusations that weigh on the Iranian fugitives ... something unprecedented and never before seen."

The document did not appear to show direct or documentary evidence of a deal, but it did include wiretap transcripts of several people discussing such negotiations and saying the deal was approved by "la jefa" — Spanish for a female "chief" — and "at the highest level."

The government dismissed Nisman's allegations as "weak" and "baseless," and Fernandez on Tuesday released a long message saying Nisman's investigation was meant "to divert, to lie, to cover up and confuse" ahead of a trial of former President Carlos Menem and other officials for a separate alleged cover-up of the bombing. Nisman, however, was the prosecutor in that case as well.

Nisman was appointed to his post in 2005 by then President Nestor Kirchner, Fernandez's late husband, after a bungled 10-year probe launched under Menem that led to a trial in which all the defendants were found innocent.