The situation came to the D.A.'s attention after a gun dealer reported suspicious purchases. The office investigated and last February arrested a woman who admitted selling guns to a group of drug dealers.
Henriquez, allegedly a member of that group, obtained a copy of the woman's statement to police, posted all eight pages of it to Facebook and called for the murder of the woman as a "snitch," Williams said.
In one post, Henriquez allegedly wrote: "KILL RATS point blank period." In other posts, he allegedly threatened the woman's family. Henriquez recently was released from jail on $25,000 bail, Williams said.
Williams said he was publicizing the situation because Facebook ignored earlier requests from the D.A.'s office to remove the threats.
Some of the content has been taken down, but Henriquez's post to "kill rats" remained on Facebook as of Monday.
A Facebook spokesman said that the site "works with law enforcement to the extent required by law and where appropriate to ensure the safety of Facebook users. We work very hard to be a good partner to law enforcement and any assertion to the contrary is false."
In his letter, Williams accuses Zuckerberg of "contributing to the culture of intimidation and fear . . . in this city."
Williams said that because his office prosecuted 1,040 cases of witness intimidation last year, it's an apparent serious problem that would benefit from Facebook's cooperation.
"This goes beyond free speech," Williams said. "When you solicit their murder, that is not free speech. That is a crime."