A new estimate says fewer than a dozen people arrested in China's sweeping crackdown on the Tiananmen Square democracy protests remain in prison 23 years later.

The San Francisco-based Dui Hua Foundation said Thursday its estimate is based on information culled from its database of prisoners and from unofficial sources. It says a source in the Chinese government confirmed that the estimate of less than a dozen is accurate.

The government has never provided a public reckoning of the military assault on June 3 and 4, 1989, that ended the democracy protests centered on Beijing's Tiananmen Square. Thousands were believed to have been detained in a nationwide crackdown that followed.

Dui Hua estimates more than 1,000 among those were ultimately sentenced to prison.