Mayor Michael Bloomberg was the target of a threatening letter that tested positive for ricin, the NYPD said May 29, 2013. View Full Caption Drew Angerer/Getty Images

NEW YORK CITY — An anonymous threatening letter to Mayor Michael Bloomberg, opened in Lower Manhattan on Friday, tested positive for the poison ricin, police said Wednesday evening.

The letter, which came from Louisiana, referenced Bloomberg's strong advocacy for gun control and threatened his life, sources and officials said.

“You’re not going to take my guns. You’ll have to kill me before you take my guns. If you take my guns, you should see what I’m going to do to you,” the letter read, according to the New York Post.

The author said they had a “constitutional and God given right and I will exercise that right til I die,” the newspaper reported.

An identical letter sent to the Bloomberg-led Mayors Against Illegal Guns coalition in Washington, D.C., received on Sunday, also tested positive for the toxin, which is potentially fatal if inhaled or ingested, according to police and the Centers for Disease Control and Prevention.

A clerk screening city mail at 100 Gold St. in Lower Manhattan opened the first ricin-containing letter on Friday and found an orange, greasy substance inside, sources said.

The clerk called the cops, and the NYPD's Emergency Service Unit responded and examined the letter, sources said. The NYPD did some preliminary tests and it was not immediately clear whether the substance contained ricin. Police called in the FBI Joint Terrorism Task Force, which sent a sample to the National Bioforensic Analysis Center in Maryland.

Preliminary tests on the sample on Wednesday indicated the presence of ricin, NYPD officials said.

Members of the Emergency Service Unit who came into contact with the letter on Friday got sick the next day with minor symptoms related to ricin exposure, including diarrhea, but they recovered quickly, officials and sources said.

No civilian personnel in New York or Washington had any symptoms after their exposure, according to the NYPD.

Ricin, which is extracted from castor beans, is more toxic than cyanide. As little as 500 micrograms, or an amount the size of a pinhead, can be fatal if inhaled or ingested, according to the CDC.

The FBI Joint Terrorism Task Force and the NYPD Intelligence Division, which is responsible for the mayor’s protection, were investigating the threats this week, authorities said.

The mayor's press office referred requests for comment to the NYPD.