Threatening letters to Bloomberg test positive for ricin

(CNN) -- 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 referenced the debate on gun laws.

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

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

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

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.

The two letters had the same postmark, a law enforcement official who had been briefed on the investigation, told CNN. Neither was sent from New York City or Washington, he said.

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.