Story highlights Suspicious substance was oily, New York police official says

Postal union says letters were postmarked in Shreveport, Louisiana

Letters were addressed to Bloomberg, one went to an organization he founded

Ricin is a toxin that can kill in a matter of days

Preliminary tests indicate ricin was found in letters sent this past weekend to New York Mayor Michael Bloomberg, New York deputy police commissioner Paul Browne said Wednesday.

Browne said the letters to Bloomberg contained a threat to the mayor and mentioned the debate on gun laws.

"The letter obviously, referred to our anti-gun efforts but there's 12,000 people (who) are going to get killed this year with guns and 19,000 that are going to commit suicide with guns, and we're not going to walk away from those efforts," Bloomberg said.

One letter addressed to the mayor's office was opened at the city government's mail facility, Browne said.

The suspicious material found in the two letters was a "pink-orange oily substance," he said, adding that it was the second of two tests that showed what appeared to be trace amounts of ricin.

The substance is being tested at the National Bioforensic Analysis Center in Maryland, with conclusive results expected by Friday.

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Some of the emergency services workers who touched the letter Friday were examined after they showed minor intestinal symptoms of ricin exposure on Saturday, Browne said. The symptoms have since subsided.

Civilian workers showed no symptoms, Browne said in a statement.

"We take a lot of security measures as you know," Bloomberg said. "The men and women that open the mail for example ... they are well trained."

The second letter to the mayor was opened by Mark Glaze, director of Mayors Against Illegal Guns -- founded and co-chaired by Bloomberg -- in Washington on Sunday. Browne's statement appeared to indicate Glaze showed no symptoms.

A spokeswoman for the organization declined to comment Wednesday.

Both letters were postmarked in Shreveport, Louisiana, on May 20, the American Postal Workers Union said on its website.

Bloomberg is an outspoken critic of current gun laws. In March, he said nationwide background checks on all gun sales would save lives.

"We know that's true, because in states that already require background checks on private sales, the rate of women murdered by an intimate partner armed with a gun is 38% lower than in states that don't have such background checks," he said.

FBI spokesman Jim Margolin told CNN the agency is working to determine from where the letters were sent and who sent them.

If inhaled, injected or ingested, less than a pinpoint of ricin can kill a person within 36 to 48 hours because of the failure of the respiratory and circulatory systems. There is no known antidote for the toxin, which is derived from castor beans.

In April, letters were sent to Obama; Sen. Roger Wicker, R-Mississippi; and Sadie Holland, a judge in Lee County, Mississippi. James Everett Dutschke of Tupelo, Mississippi, has been charged with possession and use of a biological agent in connection with the case.

Last week, FBI agents arrested Matthew Ryan Buquet after a grand jury charged him with mailing threatening communication to a senior judge in the U.S. District Court for the Eastern District of Washington state.

The FBI said in a statement that tests -- conducted by that agency and the Spokane Regional Health District -- showed that a suspicious substance found with the letter was "active ricin toxin."

There are no indications the cases are connected.