With nowhere else to turn, he called his brother, a Jesuit priest, who put him in touch with the man he credits with saving his life: Jorge Mario Bergoglio.
It was 1976, South America's dictatorship era, and the future Pope Francis was a 30-something leader of Argentina's Jesuit order. At the time, the country's church hierarchy openly sided with the military junta as it kidnapped, tortured and killed thousands of leftists like Mosca.
Critics have argued that Bergoglio's public silence in the face of that repression made him complicit, too, and they warn against what they see as historical revisionism designed to burnish the reputation of a now-popular pope.
But the chilling accounts of survivors who credit Bergoglio with saving their lives are hard to deny. They say he conspired right under the soldiers' noses at the theological seminary he directed, providing refuge and safe passage to dozens of priests, seminarians and political dissidents marked for elimination by the 1976-1983 military regime.
Mosca was 27 then, a member of a leftist political movement banned by the military government in his home country of Uruguay.
Mosca said he could hardly breathe until they had passed through the seminary's heavy iron doors, but Bergoglio was very calm.
"He made me wonder if he really understood the trouble he was getting into. If they grabbed us together, they would have marched us both off," said Mosca, who stayed hidden in the seminary for days, until Bergoglio got him an airplane ticket to Brazil.
Soldiers prowled inside the walled gardens, sniffing for fugitives. But a full raid on the spiritual center was out of the question since Argentina's dictators had cloaked themselves in the mantle of Roman Catholic nationalism. And a constant flow of people masked Bergoglio's scheming from an air force outpost next door.
Several new books assert that Bergoglio's public silence enabled him to save more people.
"Bergoglio's List," by Vatican reporter Nello Scavo, is already being developed into a movie, its title playing on the "Schindler's List" film about the Nazi businessman whose subterfuge saved hundreds of Jewish prisoners during the Holocaust.
Marcelo Larraquy, author of "Pray for Him," told the AP that Bergoglio saved "20 or 30" people. Scavo said about 100 owe him their lives.
Both authors say the full number will likely never be known, largely because Bergoglio remains so circumspect.
Like many Argentines, Bergoglio "remained silent in the face of atrocity," but he was determined to thwart the death squads when he could, said Larraquy, who runs investigations for the Argentine newspaper Clarin. "He used back channels, did not complain in public and, meanwhile, he was saving people who sought refuge in the Colegio."
"He locked them up in the compound, gave them help and food, and set up a logistical network to get them out of the country," Larraquy added. "But his condition for giving them refuge was that they had to give up all political activism."
New ways of thinking were running through the lower ranks of Latin America's Catholic Church in the 1970s, influenced by Vatican II reforms announced in 1965. Many lay workers and clergy embraced "liberation theology," which promoted social justice for the poor.
Many were politically active and some were Marxist, but others were simply committed social workers. The right-wing military made few distinctions. Priests as well as Catholic lay workers began to disappear at the hands of death squads.
Sitting in a seminary garden whose tranquility was broken only by the gurgling of a fountain and leaves rustling in the breeze, theologian Juan Carlos Scannone quietly told the AP of the terror he felt decades ago.
Scannone said he was targeted because he promoted a non-Marxist "theology of the people" and worked with slum-dwellers in the city's "misery villages." He said Bergoglio not only defended him against criticism within the church, but personally delivered his writings for publication even when the military was trying to find him.
"It was risky," Scannone said. "Bergoglio told me never to go out alone, that I take someone along so that there would be witnesses if I disappeared."
Scannone said he "wrote a lot about the philosophy of liberation and the theology of liberation, which at the time was a naughty word ... Bergoglio would read it and tell me, 'Don't mail this from San Miguel, because it could be censored,' and he would mail them from Buenos Aires with no return address."
His recollection suggests Francis' view on liberation theology may have always been more nuanced than some of his critics suggested before he became pope. Francis still draws a line against Marxism, but has helped rehabilitate some liberation theologists. The movement's founder, Gustavo Gutierrez, received applause this year during a book presentation at the Vatican.
Bergoglio also intervened, at the request of outspoken Bishop Enrique Angelelli, to save three seminarians after Catholic lay workers were killed in western La Rioja province in 1976. The seminarians were being followed by the same death squads and accused of being "contaminated with Marxist ideas." No one else would take them. Bergoglio was able to rescue Mario La Civita, Enrique Martinez and Raul Gonzalez just as Angelelli was assassinated in August 1976. "I watched him save lives," La Civita recalled. "It was a difficult time because two or three soldiers were always walking around in the back of the compound. Bergoglio had a strategy of generating confidence in them so that they wouldn't think he had people hidden." But Bergoglio couldnt save everyone he tried to help.
Esther Ballestrino de Careaga, a communist who had been Bergoglio's boss in a laboratory before he became a priest, pleaded with him to hide the Marxist literature in her house after her daughter was kidnapped and son-in-law disappeared. "Those were the books that Bergoglio fought (against), but he carried them away anyway," Larraquy said.
A short while later, she co-founded the Mothers of the Plaza de Mayo, publicly demanding that the junta account for the missing. Soon, she disappeared.
Bergoglio's role was more ambiguous in the case of two slum priests, Orlando Yorio and Francisco Jalics. He supported their social work, but not their political activism, much less their contacts with armed revolutionaries, and he made them quit the Jesuit order, leaving them without church protection, Larraquy said.
"Bergoglio told them to abandon their political project in the slum, and they refused; they were insubordinate," Larraquy said.
Yorio, Jalics and some Catholic lay workers were seized a short time later after holding Mass, and taken to the regime's clandestine torture center inside the Navy Mechanics School.
Bergoglio testified as part of a human rights trial in 2010 that he persuaded another priest to fake an illness so that he could hold a private Mass for dictator Jorge Videla and personally plead for the Jesuits' release. They were set free in October 1976, left drugged and blindfolded in a field.
"Bergoglio contributed by helping the persecuted, and he dedicated himself to obtaining the release of his kidnapped priests. Still, he didn't participate at the time in the fight against the military dictatorship in defense of human rights," said Adolfo Perez Esquivel, whose human rights work in Argentina won him the Nobel Peace Prize. Very few other detainees emerged alive from the Mechanics School, and when Bergoglio testified, he didn't reveal any new details about the others who disappeared, "even when their families are demanding an answer," complained human rights lawyer Myriam Bregman.
Bregman says Francis should clear up doubts by opening the church's archives. "We've asked for it and we keep waiting. The church was part of the dictatorship, it was a direct accomplice, and today it continues without revealing all that it has in its archives," Bregman said.
Mosca sides with Bergoglio. Referring to Yorio and Jalics, Mosca said: "He did not hesitate in risking everything for my cause. He didn't know me. If he did all that for me, how much would he have done for those two?"
Associated Press writer Nicole Winfield at the Vatican contributed to this report.