SANTIAGO, Chile -- Olga Weisfeiler has spent 17 years following clues to find out what happened to her brother, a U.S. citizen and mathematics professor who disappeared under general Augusto Pinochet's brutal dictatorship in Chile.

She has written countless letters to U.S. lawmakers and hired a lawyer to work on the case, but never got any closer to the truth. Still, Olga has not given up hope and has started a new round of letter writing, this time to Chilean officials, she told Reuters.

Boris Weisfeiler, a Russian-born Jew, disappeared in 1985 as he was trekking in southern Chile near a secretive German enclave, Colonia Dignidad, known to have collaborated with Pinochet's secret police.

He remains the only American that disappeared during the 17-year-long military rule and whose whereabouts remain a mystery.

"He disappeared in Colonia Dignidad. I believe, I want to believe he is still alive and being held as a prisoner," Olga Weisfeiler said in a telephone interview from her home in Newton, Massachusetts.

She sent letters a few weeks ago to Chilean Defense Minister Michelle Bachelet and Army Chief Emilio Cheyre in the belief that the Chilean military is withholding vital information about her brother's disappearance and probable death.

"I ask you to use your position to investigate Boris' case and order maximum cooperation from the military with the judge's investigation and to release the documents that contain information about the kidnapping of my brother and his detention in Colonia Dignidad," Olga's letter read.

She Has Received No Reply.

Any eventual answers, though, will possibly come from Judge Juan Guzman, the magistrate who is handling that case and 300 other charges of human rights violations against Pinochet.

Torture survivors have described the horrific interrogation methods used inside the colony 215 miles (350 km) south of Santiago, and many believe remains of victims are buried nearby.

An official report on human rights abuses under Pinochet names Colonia Dignidad as one of several secret torture camps where suspected leftists were taken and possibly executed.

The colony was founded as a religious sect in 1961 by former Nazi nurse Paul Schaefer and functioned as a virtual state within a state under Pinochet. Paul Schaefer has been on the run from the Chilean authorities for the past four years on charges of sexually abusing young boys from the colony.

About 300 people, mostly of German descent, still live in Colonia Dignidad, which is circled by a high-tech security system.

Guzman has entered the compound with a search warrant but has been unable to find any evidence.

Mistaken for a Spy Olga Weisfeiler believes her brother, a lecturer at the University of Pennsylvania, was captured by the military, which mistook him for a spy and later took him to Colonia Dignidad.

According to declassified U.S. State Department documents, Chilean military officers confessed to the U.S. consulate a couple of years after Weisfeiler's disappearance that the American was detained and tortured in Colonia Dignidad.

There is no evidence of his actual death.

"They most likely were confused. Since he carried an American passport, they thought he was a U.S. spy...They saw he had been born in Moscow so they accused him of being a Russian spy," Hernan Fernandez, Olga Weisfeiler's lawyer, told Reuters.

"After that, according to the military officer interviewed by the consulate, he was taken to Colonia Dignidad, where he was brutally tortured. Once the Germans had finished torturing him, they told the military he was a 'Jewish dog'...The patrol then left, leaving Boris there," said Fernandez.

Although Weisfeiler's case has not raised as much media attention as that of American journalist Charles Horman, whose death in Chile inspired the movie "missing," it has nonetheless sparked concern among U.S. diplomats as he remains the only U.S. citizen to "disappear" under Chilean military rule.

Olga Weisfeiler has been in Chile only once, but she has a shared pain with more than 1,000 Chilean families who lost a relative.

"I share the same open wound of not knowing what happened, the anxiety of not knowing why it happened and the desperation of an endless search, not knowing whether I will ever see my brother again," she said.

Like the Chileans, she is still awaiting an answer.