Philadelphia District Attorney Seth Williams says Facebook is refusing to take down posts that threaten a witness in a weapons case.

A 20-year-old Kensington man was charged in December with using the social-networking site to threaten a woman who was testifying against his friends in a case involving straw purchases of firearms.

The site has so far refused to remove posts Freddie Henriquez wrote declaring that he wanted to "kill rats," Williams said. Henriquez also posted the eight-page statement the woman gave police on his Facebook page, he said.

Williams said he is sending a letter to CEO mark Zuckerberg asking for the posts to be removed after the site denied previous requests.

"On every attempt, they said 'no,'" Williams said.

He said Facebook's refusal violates the site's own policies on bullying and harassment.

Facebook's terms of service say users "will not bully, intimidate, or harass any user" or "use Facebook to do anything unlawful, misleading, or discriminatory."

"The posting of the witness's statement and references to 'rats' and 'kill rats' are clearly intended to intimidate and harrass," Williams' letter to Zuckerberg says.

Facebook's press office did not immediately respond to a request for comment.

Williams said the site has not explained why it determined that the posts don't violate its policies.

The witness Henriquez is accused of threatening was arrested in February 2012 as part of a conspiracy to straw-purchase firearms for drug dealers, authorities said. The woman agreed to cooperate and testify against the dealers; one of those men gave Henriquez a copy of her statement, which he posted on Facebook in November, prosecutors allege.

"He posted the statement for the entire world to see, potentially placing this witness in a grave danger from anyone who has an ax to grind with those who cooperate with the authorities in a criminal case," Williams wrote to Zuckerberg.