Anonymous letters sent to New York City Mayor Michael Bloomberg and the director of the organization Mayors Against Illegal Guns, in Washington, D.C., contained material that initially tested positive for ricin, according to NYPD Deputy Commissioner Paul J. Browne.

According to a police statement released Wednesday, the anonymous letters were opened in New York on Friday at the city's mail facility in Manhattan and in Washington on Sunday at an office used by Mayors Against Illegal Guns -- a nonprofit founded by Bloomberg that lobbies for stricter gun control. It is believed that both letters threatened the mayor and made reference to the gun control debate.

The postal workers' union, citing information it got in a Postal Service briefing, said the letters bore a Shreveport, La., postmark.

Louisiana State Police spokeswoman Julie Lewis said state authorities have deferred to the FBI and have not opened an investigation. The Shreveport postal center handles mail from Louisiana, Texas and Arkansas, so the letter might have come from any of those states, Lewis said.

Both the letters contained an oily pinkish-orange substance, Browne said.

Civilian personnel who came into contact with the letters were not presenting symptoms of ricin exposure. However, three officers who came into contact with the letter sent to Bloomberg experienced some minor symptoms that have since abated.

Police said the letter in Washington, D.C., was opened by Mark Glaze, the director of Mayors Against Illegal Guns. He was working out of a lobbying firm office.

The incidents are being investigated by the FBI Joint Terrorism Task Force and the NYPD Intelligence Division.

"In terms of why they've done it, I don't know," Bloomberg said at an event Wednesday night. One of the letters "obviously referred to our anti-gun efforts, but there's 12,000 people that 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," said Bloomberg, adding that he didn't "feel threatened."

Bloomberg and Boston Mayor Thomas Menino founded Mayors Against Illegal Guns, which now counts more than 700 mayors nationwide as members. It lobbies federal and state lawmakers, and it aired a spate of television ads this year urging Congress to expand background checks and pass other gun-control measures after the school shooting in Newtown, Conn. The background check proposal failed in a Senate vote in April, and other measures gun-control advocates wanted -- including a ban on sales of military-style assault weapons -- went by the wayside.

Ricin can be fatal in small doses if inhaled or swallowed. According to the Centers for Disease Control, ricin is a poison found naturally in castor beans. Symptoms can include difficulty breathing, vomiting and a redness on the skin depending on how the affected person comes into contact with the poison.

News of the letters comes as a 37-year-old man is charged in Washington state with sending the toxin in letters to a federal judge, and about a month after letters containing the substance were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man was arrested in that case.

The Associated Press contributed to this report.