The Kirchner government has all but accused U.S. intelligence services of sheltering their most-wanted man, putting a new strain on relations that are already pretty sour.
Anibal Fernandez, Kirchner's chief of staff, told reporters recently that Washington had failed to respond to eight requests sent by his government seeking information on Stiuso's whereabouts.
"We ask ourselves sometimes: 'Is the United States ready to allow the bilateral relations between it and Argentina to worsen for a man they all say has no importance, no strategic value for the United States?' " he said.
But Stiuso is also a figure at the center of the most sensational murder investigation in recent Argentine history.
Kirchner alluded to Stiuso in her address to the United Nations General Assembly last month, saying "he is being protected."
"Protected from what? I don't know," Kirchner said.
But an attorney for Stiuso said his client fled Argentina in fear of his life a month after Nisman turned up on the floor of his bathroom Jan. 18 in a pool of blood. Next to him was a pistol he'd allegedly borrowed from an associate the day before.
A hard-charging federal prosecutor, Nisman had spent two decades investigating the worst terrorist attack in Argentine history: the 1994 bombing of the AMIA Jewish community center in Buenos Aires that killed 85.
Iranian agents are suspected of carrying out the attack, and on the eve of his death, Nisman was preparing to present an explosive report accusing Kirchner of attempting to cover up Iran's role as part of a secret deal with Tehran.
Kirchner's government quickly declared Nisman's death a suicide. But as protests erupted against her government, Kirchner said Nisman had been the victim of a diabolical plot by Stiuso to smear her and destabilize the country.
The once-powerful intelligence chief who allegedly presided over a vast wiretapping apparatus prior to his removal by Kirchner last December was said to be working closely with Nisman as the prosecutor prepared his report.
Stiuso has not been formally charged in Nisman's death, but he is wanted for additional questioning by investigators, and faces an array of charges including money laundering and obstructing the investigation of the 1994 bombing.
Given his contacts with Nisman and knowledge of state secrets, Stiuso may be the one person who would know if the prosecutor was killed or took his own life, and whether his damning accusations against Kirchner might be true.
The U.S. Embassy in Argentina declined to comment, issuing a statement saying it responds through established judicial channels.
Argentines will vote for president Oct 25, and the Nisman death and the Stiuso affair have not been a major theme of the race. The contest to succeed Kirchner is primarily between her party's candidate, Daniel Scioli, governor of Buenos Aires province, and opposition leader Mauricio Macri, mayor of the city of Buenos Aires. Polls suggest a November runoff between the two men is likely.
Meanwhile, the search for Stiuso and the government's escalating accusations against the Obama administration have been a daily theme in Argentine news coverage this month.
Reporters from Argentina's La Nacion traveled to Miami last week attempting to locate Stiuso at the addresses provided by the Kirchner government to Interpol. They found no trace of the ex-spymaster, nor any Argentines who said they had seen him.