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On March 14, 1969, University of Colorado Russian history professor Thomas Riha went to a dinner party at a friend’s house. He stayed late and walked out to his car around 12:30 a.m. for the short drive across town to the house he’d shared until recently with his soon-to-be-ex-wife. Riha, an amiable, bookish 40-year-old who had moved to Colorado two years earlier, was never seen again.
To this day, no one knows for sure what happened—but theories abound, and you can explore them for yourself at a new History Colorado exhibit. The Disappearance of Thomas Riha (April 25 through February 2026) invites visitors to browse dozens of artifacts pertaining to the mystery, including photos, journals, declassified government documents, and court records. At the end of the exhibit, people are asked to provide their best guess as to what happened.
“It’s a unique Colorado cold case with international ramifications,” says Katherine Mercier, the exhibit’s lead curator. “There was intense public interest and speculation of possible espionage…. And the disappearance even ended up on the desk of [then FBI director] J. Edgar Hoover.”
Below, four of the most puzzling aspects of this stranger-than-fiction Cold War mystery.
1. At first, people assumed Riha’s disappearance had something to do with the Cold War.
Initial speculation in the media, and from friends and colleagues of Riha, focused on a possible link between Riha and the Soviet Union. It’s not hard to see why. Riha, an expert in Russian history who spoke five languages and grew up in Czechoslovakia, was a U.S. Army veteran who had served at the military’s Psychological Warfare Center. He lived in Moscow for a year as an exchange student; he and his fellow students were briefed by the FBI before and after their trip, and they were followed by KGB agents (Russian secret police) during their time overseas. The FBI even briefly considered trying to recruit Riha as a double agent, journalist Eileen Wellsome wrote in her 2021 book Cold War Secrets. But there’s no evidence that Riha was ever involved in espionage or that his disappearance was connected to Russia in any way.
2. The theory that his shady friend (or maybe lover) was involved seems a lot more compelling.
While he was teaching at the University of Chicago, Riha befriended a woman named Galya Tannenbaum, and she followed him to Colorado. She later said they lived together and were romantically involved. Tannenbaum went by the nickname “the Colonel” and lied about her past, saying that she had served in the Army. In reality, she had a long history of mental illness, went to prison for forgery and embezzlement, and assumed several fake identities.
A week before Riha disappeared, his wife, a woman by the name of Hana Hruska, reported a bizarre series of events: She said that Tannenbaum had tried to coerce her to sign a document and take pills. She didn’t know what the document was or what the pills contained, so she refused. After that, Tannenbaum tried to poison or incapacitate her with ether, a sweet-smelling drug used as an anesthetic, but Hruska escaped, she told her attorney.
Just a few months after Riha vanished, two of Tannenbaum’s other friends—one of whom had recently named her as a beneficiary in his will—died from cyanide poisoning. Their deaths were ruled suicides, but the police became suspicious, especially since a document had been found in which Riha appeared to sign his house over to Tannenbaum. When the cops searched Tannenbaum’s home, they found a pound of cyanide (which, while damning, apparently wasn’t strong enough evidence to charge her with homicide). She was charged with forgery and found not guilty by reason of insanity. Tannenbaum committed suicide by cyanide at the Colorado State Hospital in Pueblo in 1971.
Read More: 5 of Colorado’s Wildest True Crime Stories
3. The case led to a major conflict between the FBI and the CIA.
Riha’s disappearance prompted a communications breakdown between the two federal agencies. “There were a lot of questions about where [Riha] went, and the FBI shared information with the CIA, and then the CIA shared information with the public,” says Mercier. “Then everyone denied they’d shared information with anyone.” As was later made public in a 1975 Senate investigation, FBI director J. Edgar Hoover became so angry that he cut off contact between his agency and the CIA. While the breakdown was fairly brief, it wasn’t a good look for the feds.
4. The exhibit illustrates the crucial role Colorado played in the Cold War.
“If you live in Colorado, you live surrounded by Cold War history,” Mercier says. “We were a hub of Cold War activities because we were in the middle of the U.S., so we were considered to be a safer place in the event of nuclear warfare.” Plus, ample land was available for government and military efforts, from the Rocky Flats nuclear weapons plant in Denver to the Rocky Mountain Arsenal in Adams County. Multiple Air Force bases, missile silos, and weapons manufacturing facilities all put Colorado at the center of the nation’s Cold War effort. “It was at the forefront of everyone’s minds,” Mercier says—so it made sense that when Riha disappeared, his friends and coworkers immediately wondered if he’d been abducted by the KGB.
Her personal favorite artifact in the exhibit, Mercier says, is a lapel pin that reads “I’m prepared. Are you?” It’s one of several items on display that isn’t specific to the Riha case, but gestures more broadly to the atmosphere of fear and paranoia that pervaded American life at the time.
No matter what you think happened to Thomas Riha—and at the end of the exhibit, guests get to “vote” by writing their thoughts in chalkboard paint on the wall—that makes for an interesting parallel with our current political moment.
