Source?

One of the most striking things I have witnessed has been the letters that young protesters have been writing in case they are arrested or disappeared by security services. These “last letters,” to be read in the event that they don’t return from the protests, explain to their families why they are protesting, and also serve as a “no-suicide declaration.” Nine thousand, two hundred, and sixteen people have been arrested during the 12 months of protests, a figure much larger than the total prison population of 7,023. This discrepancy, combined with a series of mysterious deaths and disappearances that have been explained as suicides or accidents, has made protesters wary of their own safety. So the letters serve to say, “I will not be another suicide statistic.” As one young person wrote in their letter, “I have no regrets, not for a moment; even though missing or dead, I will never stay silent in the face of injustice”

From US State Department itself on the situation in Hong Kong [0].

>There were no credible reports that the government or its agents committed arbitrary or unlawful killings.

This is China-Hawk Mike Pompeo's state department BTW, report released in 2020 well after protests ended. If Pompeo couldn't find mystery murders, it probably doesn't exist.

https://www.state.gov/reports/2019-country-reports-on-human-... [0]

[1] “Activist resurfaces to clear 'mystery death'”

https://www.thestandard.com.hk/section-news/section/11/22246...