The last time Olga Weisfeiler spoke to her brother Boris was in late 1984, just before he flew to Chile for a hiking holiday.
"I need to get away from all this snow in Pennsylvania," he told her. "I'm heading south. It's summer down there."
Days later, Boris Weisfeiler was stepping out into the foothills of the Andes mountains in southern Chile, a rucksack on his back and an open trail ahead of him.
But he never made it home.
In January 1985, his backpack was found by the side of a river. Mr Weisfeiler, who was 43, has not been seen since.
A Chilean investigation that same year concluded that Mr Weisfeiler, an experienced hiker, had drowned while crossing the river.
But his body was never found.
Then, in 2000, declassified US documents suggested Mr Weisfeiler might have suffered an altogether more sinister fate.
The documents cited a witness who claimed to have been part of a Chilean army patrol that seized Mr Weisfeiler and took him to Colonia Dignidad, a secretive Germanic agricultural commune set up in Chile in the 1960s.
It is known that Gen Augusto Pinochet's military dictatorship, which ruled Chile from 1973 to 1990, occasionally used Colonia Dignidad as a torture and detention centre.
The witness said Mr Weisfeiler had been interrogated at Colonia Dignidad before being forced to kneel and "murdered with a shot in the nape of his neck".
Although the allegation has never been proved, it helped lead to a new investigation.
In 2012, a Chilean judge charged seven former police and military officers with kidnapping Mr Weisfeiler, and an eighth man with being an accomplice to the crime.
Mr Weisfeiler's family hoped justice would finally be done.
But now, against all expectations, the judge has closed the case.
In a ruling last month, he said that although he accepted Mr Weisfeiler had been abducted, he was the victim of a common crime rather than a human rights violation and, as such, the case was subject to a statute of limitation.
All eight men are free.
"I'm devastated," Mr Weisfeiler's 72-year-old sister Olga told the BBC in Santiago.
"I've spent the last 16 years trying to find out what happened to Boris, and now this!"
The US embassy in Chile described the ruling as "a frustrating setback".
"It is vital that victims and family members are able to seek justice in human rights violations, no matter when they were committed," it said in a statement.
The judge's decision has provoked dismay within scientific circles.
Boris Weisfeiler was a professor of mathematics at Pennsylvania State University and a gifted academic.
The Committee of Concerned Scientists, a US-based body which defends the freedom of scientists around the world, urged Chile's President Michelle Bachelet to intervene and order a re-opening of the case.
"Dr Weisfeiler deserves better," the Committee said in a letter to Ms Bachelet.
"On behalf of his family, we urge your government to return to the good work of building a culture of respect for human rights in Chile."
The Chilean Mathematical Society also urged the authorities to reopen the case.
It described Mr Weisfeiler as "a brilliant mathematician who, at the time of his disappearance, was at the zenith of his career".
Olga Weisfeiler, backed by the US embassy, has lodged an appeal against the judge's ruling.
"If we lose, I'll go to the Supreme Court and if we lose there I'll go to international courts," she vowed. "I can't let this go."
For now though, she and her family are no closer to finding out what happened to Boris Weisfeiler that January, more than 30 years ago, in the hills of southern Chile.
"I have countless sleepless nights wondering what really happened to my beloved brother," she said.
"I am still waiting for the answers as to why, where and when Boris lost his life."