Police say the letter was handwritten in block letters and made a threat related to Weiner's vote in favor of the sweeping federal health care legislation.
Workers were decontaminated as a precaution. More sophisticated lab testing was also to be done to determine the substance.
Nine people in the office were being checked out, although none were reported to be injured.
It was the latest in a string of several threats made against Weiner and apparently linked to the health care vote.
At least three other area lawmakers said they have also received threats based on their health care reform vote.
Rep. Carolyn Maloney received threatening letters that have been turned over to Capitol police. Rep. Eliot Engel received threatening faxes and calls. Congressman Steve Israel said he received anti-semitic letters.
Weiner has also received profanity laced letters with anti-Semitic rhetoric. One had a swastika and called him "Schlomo Weiner," among other things.
Weiner brushed off the incidents, saying in an interview today that his "leading with my chin" on the health care debate made him "something of a lightning rod."
"Since the time of the passage of the bill was imminent to today, there has been an increase of these things," Weiner admitted, without getting into specifics.
"I think we all bare a responsibility to make sure that its one thing to have a debate about an issue, quite another to say things that imply that being violent, anti-Semitic, racist, homophobic, is somehow sanctioned," Weiner said.
Weiner said he was concerned about the people working in his community offices.
"We still very much want to be an office that is open to our constituents, but we are also taking some extra safeguards," he said.
(The Associate Press contributed to this story.)