BEIJING -- Twenty-five years ago this week, China's military crushed the pro-democracy protest in Beijing's Tiananmen Square. To this day, no one knows how many were killed.
One image stands out: that of a single protester, carrying shopping bags, blocking a column of tanks. His victory was brief, and the crackdown continued.
A quarter century later, China's leaders still won't let anyone talk about it.
When CBS News tried to interview activist You Weijie, the widow of a man who was shot and killed near Tiananmen Square in 1989, a woman who would not identify herself called the police.
You said she was being watched by police and could not leave her house, so CBS News went to her. No one came to the door, but five policemen showed up outside.
Chinese security prevented CBS News from speaking to a number of people on the anniversary, so we traveled to Hong Kong -- beyond the reach of mainland police.
There, we met former protester Lee Cheuk-Yan, who has founded a new museum about Tiananmen.
Lee was in Beijing the night of the crackdown. He said he thought the pro-democracy movement had momentum, but then the tanks rolled in. He remembered the gunfire and the fear when the lights went out.
"That's the darkest point of my life, when the whole Tiananmen Square lights were switched off," he said. "In the morning, when the lights came out, you could see bodies already being transported to the hospital."
Twenty-five years later, Lee's museum is waging a new protest against the erasing of history.
"In China, no one (is) allowed to discuss about June 4, you know, parents do not talk to their children about that, so there's a vacuum there," he says.
Many visitors to the museum come from mainland China, including Jiang Luo.
"I didn't know about it when I was like, middle school and high school, because it wasn't really mentioned in the textbook," Jiang says.
Asked why the historical event is so sensitive 25 years later, Lee says, "That's exactly the question I want to ask the Communist Party: Why are you so afraid?"
Today's Communist government is afraid of what Lee says is simply an historical truth.
Beijing is a city on lockdown. CBS News has seen security beefed up across the city -- and certainly at its center in Tiananmen Square. CBS News' own Chinese colleagues have come under pressure from authorities, as this is an anniversary they do not want to remember.