Story highlights The FBI says four similar letters were postmarked the same day, with similar ink 
 
 Tests show three of those letters had ricin; tests are pending on the fourth one 
 
 The FBI is looking for a letter that was directed to the CIA 
 
 All five letters are believed to have been sent from Spokane around May 13 
 
 The FBI announced Thursday its agents are looking for a letter, this one directed to the Central Intelligence Agency, that it believes was similar to at least three other letters that contained ricin. 
 
 Letters were sent to a federal judge, a Spokane post office, President Barack Obama and Fairchild Air Force Base, the FBI said. Tests are being conducted on the letter sent to Fairchild Air Force Base to determine whether it -- like those other three letters -- contained ricin. 
 
 The letter sent to the CIA was "addressed in a manner consistent with the first four envelopes" and went to "a location that does not receive mail deliveries." 
 
 As of Thursday evening, authorities have not been able to locate this letter. It was not immediately clear how they knew about its existence. 
 
 Eight days ago, 37-year-old Matthew Ryan Buquet was arrested as part of the investigation into the aforementioned letter sent to U.S. District Court Judge Fred Van Sickle. That letter contained a message threatening to kill and injure the judge. 
 
 Both that letter and one sent to a Spokane post office were intercepted on May 14, while the letters to Obama and Fairchild Air Force Base were tracked down on May 22. 
 
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 The four envelopes that authorities have in their possession were all postmarked out of Spokane on May 13, the FBI said. Each one of them had addresses handwritten in red ink. 
 
 The bureau said it believes the fifth letter -- written to the Central Intelligence Agency at its headquarters in McLean, Virginia -- was likely postmarked the same day and also had red ink on its envelope. 
 
 The FBI's announcement Thursday out of Washington state came the same day officials intercepted a letter addressed to Obama. 
 
 The off-site facility that screens mail addressed to the White House turned the letter over to the FBI Joint Terrorism Task Force for testing and investigation, U.S. Secret Service spokesman Brian Leary said. 
 
 This letter was similar to threatening letters sent to New York Mayor Bloomberg and the gun-control group he founded. Preliminary tests indicate ricin was in those latter letters, New York Deputy Police Commissioner Paul Browne said Wednesday. 
 
 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. 

 A person of interest is being questioned in connection with threatening letters sent to President Barack Obama, New York City Mayor Michael Bloomberg and his anti-gun group, according to law enforcement officials. Jonathan Dienst reports. (Published Friday, May 31, 2013) 
 
 A person of interest is being questioned in connection with threatening letters sent to President Barack Obama, Mayor Bloomberg and the mayor's anti-gun group, according to law enforcement officials. 
 
 Authorities were questioning the Texas man to see if he had information related to the threatening letters, which were similar in content and bore the same postmark on the envelopes. Two of them, sent to Bloomberg and Mark Glaze, the director of the anti-gun group, have tested positive for poisonous ricin. Officials were testing the letter addressed to Obama to see if it contained ricin as well. 
 
 Ricin-Tainted Letter Sent to Mayor Bloomberg, Anti-Gun Group 
 
 New York City Mayor Michael Bloomberg has become the latest target of a ricin letter scare. Chief Investigative reporter Jonathan Dienst reports. (Published Thursday, May 30, 2013) 
 
 The man is described as an Army veteran who is a contractor with the Department of Defense. He has connections to Shreveport, La., where the letters were postmarked, but is not considered a suspect at this time, several officials said. 
 
 The interview with the man is just one possible lead in the case, and officials emphasize the development is still part of the early stages of their investigation. 
 
 The threatening letter mailed to Obama was received Wednesday at an off-site facility and did not reach the White House, according to the Secret Service. 
 
 "This letter has been turned over to the FBI Joint Terrorism Task Force for testing and investigation," said Brian Leary, a Secret Service spokesman. 
 
 The text of the mailings threatened: "what's in this letter is nothing compared to what I've got planned for you," police and law enforcement sources said. A photo of the letter sent to Bloomberg, obtained by NBC 4 New York, is pictured below. 
 
 That letter was discovered at City Hall's mail sorting facility at 100 Gold St. on Friday, a law enforcement source told NBC 4 New York. It appeared to contain a pink, oily substance when a mail worker came across it and was immediately flagged as suspicious. 
 
 An initial field test didn't bring up any sign of ricin, a source said. But more preliminary testing Wednesday showed the letter tested positive for ricin. 
 
 An identical letter containing ricin was sent to the Mayors Against Illegal Guns headquarters in Washington, D.C. and was opened on Monday, police said. 
 
 The mailings put New York City officials on edge; Police Commissioner Ray Kelly said other suspicious letters were investigated at downtown locations, including two at the office of the city's public advocate, who is a Democratic mayoral candidate, and another near City Hall. Those were said to be unrelated to the ricin scare. 
 
 According to law enforcement sources, the three similar threatening letters were postmarked May 20 and sent without a return address or signature. Sources said the NYPD tried to pull fingerprints off the letter discovered at the city's mail sorting site, but nothing usable was found. Authorities also plan to check for any possible DNA. 
 
 It wasn't known if any viable forensic evidence was discovered on the letters sent to Washington or the White House. All three mailings read in part: "You will have to kill me and my family before you get my guns. Anyone wants to come to my house will get shot in the face. The right to bear arms is my constitutional God given right and I will exercise that right till the day I die." 
 
 Bloomberg is the founder of the anti-gun group and has emerged as one of the country's most potent gun-control advocates, able to press his case with both his public position and his private money. 
 
 On Wednesday, the mayor said he didn't know what pushed whoever sent the ricin-laced letters to do so, but that his anti-gun efforts would not be deterred, despite the threats in the letter addressed to him. 
 
 "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." 
 
 Civilian workers who came into contact with the letters have not shown symptoms of being poisoned by the ricin, police said. Some members of NYPD's emergency service unit who did come into contact with the opened letter in New York initially showed some minor symptoms of ricin exposure, but the symptoms have since abated. 
 
 Bloomberg visited mailroom employees Thursday and thanked them for their service, a spokesman said. 
 
 According to the Centers for Disease Control, ricin is a poison found naturally in castor beans. Symptoms can include difficulty breathing, vomiting and redness on the skin depending on how the affected person comes into contact with the poison. 
 
 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. Separately, Bloomberg also has made political donations to candidates who share his desire for tougher gun restrictions. His super PAC, Independence USA, put $2.2 million into a Democratic primary this winter for a congressional seat in Illinois, for example. Bloomberg's choice, former state lawmaker Robin Kelly, won the primary and the seat. 
 
 Copyright Associated Press / NBC New York
– Officials are questioning a Texas Army veteran over ricin-laced letters sent to President Obama, Michael Bloomberg, and Bloomberg's gun-control organization, law enforcement sources tell NBC New York. The man, who works as a Pentagon contractor, isn't a suspect in the case at this point, they say. He does, however, have links to Shreveport, Louisiana, where the letters were postmarked. Meanwhile, the text of the letters is emerging: "What's in this letter is nothing compared to what I've got planned for you," they all say, per NBC New York. "You will have to kill me and my family before you get my guns. Anyone wants to come to my house will get shot in the face," the letters say. "The right to bear arms is my constitutional God given right." More suspicious letters are also under investigation in New York City, though they're reportedly not tied to ricin concerns. Meanwhile, the FBI is looking for yet another letter tied to the ricin scare, this one addressed to the CIA. Earlier this month, authorities found three similar letters containing ricin directed to Obama, a federal judge, and a post office; one sent to Fairchild Air Force Base is being tested for the substance. Matthew Ryan Buquet, 37, was arrested in connection with the letter to the judge, CNN reports.