In 1941, as Nazi German troops swept through Soviet-era Ukraine, Josef Stalin's secret police blew up a hydroelectric dam in the southern city of Zaporizhzhya to slow the Nazi advance.
The explosion flooded villages along the banks of the Dnieper River, killing thousands of civilians.
As Europe marks its Day of Remembrance for Victims of Stalinism and Nazism on August 23, a handful of Zaporizhzhya residents are battling for the recognition of the little-known wartime tragedy.
The day, which is also known as Black Ribbon Day
outside Europe, coincides with the anniversary of the 1939 Molotov-Ribbentrop Pact of nonaggression between Nazi Germany and the Soviet Union.
Ukraine suffered heavy losses both during World War II and under Stalin.
The Zaporizhzhya events took place in August 1941. As Nazi troops approached the city, Moscow sent in agents from the NKVD, the predecessor of the KGB, to blow up the city's DniproHES hydroelectric dam.
The team successfully carried out its secret mission -- which historians say was ordered by Stalin himself -- tearing a hole in the dam and temporarily cutting off part of the city from the invaders.
But the explosion also flooded villages and settlements along the Dnieper River.
The tidal surge killed thousands of unsuspecting civilians, as well as Red Army officers who were crossing over the river.
Since no official death toll was released at the time, the estimated number of victims varies widely. Most historians put it at between 20,000 and 100,000, based on the number of people then living in the flooded areas.
'People Were Screaming'
Survivor Oleksiy Dotsenko says the Dnieper turned red that day.
His account, recorded four years ago by the television channel 1+1, is one of the last remaining testimonies of the tragedy.
"People were screaming for help. Cows were mooing, pigs were squealing. People were climbing on trees," he recalled.
WATCH: Archival footage of the power station attack
Many Zaporizhzhya residents, however, are still unaware of the disaster.
Local historians and rights activists accuse city authorities of perpetuating Soviet-era efforts to cover up the truth by refusing to honor the victims.
Officials acknowledge that innocent civilians died but defend the dam's destruction as a necessary measure that helped save countless lives.
"There was no one at the time to defend Zaporizhzhya," says Oleksiy Baburin, the head of the Ukrainian Communist Party's regional branch. "We had very few soldiers. There were almost no NKVD troops or military regiments who could have stopped the Germans. This is why blowing up DniproHES allowed for the evacuation to continue."
But a number of historians reject such claims, insisting that the operation was poorly timed and that Nazi troops had no immediate plans to seize the city.
No Official Recognition
Historian Vladyslav Moroko says the men in charge of the mission, Boris Epov and Aleksandr Petrovsky, rushed the dam's explosion due to their fear of Stalin.
"In reality, Epov and his subordinates were concerned less by the possible German invasion of Zaporizhzhya than by the fact that they may not be able to carry out Stalin's order," Moroko says. "They were afraid that DniproHES would be captured and that they would not be able to carry out Stalin's order."
A monument close to the hydroelectric station, which is still in use, pays tribute to the troops that defended the facility during World War II.
A group of local residents this year put up a commemorative wooden cross in Zaporizhzhya on August 18, the anniversary of the DniproHES tragedy.
But there is still no official monument or plaque in the city to honor its victims.
Moroko and others have written an open letter urging city authorities to right this wrong. The letter went unanswered.
"This petition was public. Civil organizations and citizens responded to it and expressed their support," Moroko says. "But the government is acting like it never happened."
Written by Claire Bigg based on reporting by RFE/RL Ukrainian Service correspondent Dmytro Moroz