* Man in Texas being questioned about letters to Obama, Bloomberg
* Five threatening letters sent earlier from Washington state
* Ricin letter to CIA detected but not found, says FBI (Adds Shreveport postmark, additional detail on CIA letter)
By Mark Hosenball
WASHINGTON, May 31 (Reuters) - U.S. authorities are investigating a Texas man over threatening letters containing a deadly poison mailed to U.S. President Barack Obama and New York Mayor Michael Bloomberg, a law enforcement official said on Friday.
The probe comes as FBI officials continue investigating a separate batch of ricin-laced letters sent earlier this month from Washington state to the president and four other targets, including the CIA and a military facility.
At this point, investigators do not think the two cases are connected, the source said.
FBI investigators in Texas are questioning a man in New Boston, Texas after his wife called the police to report suspicious activity, the law enforcement source said. The agency is proceeding cautiously, and the man has not been charged, the source added.
Those letters were postmarked May 20 from Shreveport, Louisiana, according to an official briefed on the investigation. They referred to the debate over the nation's gun laws and also were sent to Bloomberg's gun control group in Washington, D.C. Initial tests showed they contained ricin. ID:nL2N0EA26I]
In the Washington state case, the FBI said on Thursday that three of the five letters, including the letter to Obama, have been intercepted and contained ricin.
A fourth letter to Fairchild Air Force Base near Spokane has been located, and a fifth letter to the CIA was detected but not found, the FBI's Seattle office said in a statement.
"The CIA letter was detected during the mail process but it was sent to a facility that does not receive mail deliveries," said Ayn Dietrich, a spokeswoman for FBI Seattle.
"We can't offer more granularity into detection methods," she said when asked what it meant for the letter to be "detected."
Authorities have already charged a Washington state man, Matthew Ryan Buquet, for allegedly mailing one of the five letters to a U.S. district judge in Spokane. On Thursday, the FBI said that letter is similar to the other four letters from Washington under investigation.
"We are aware of similarities between the five letters listed in the press release. The Shreveport letters are separate from this Spokane investigation," Dietrich said.
Mail to top public officials is not delivered directly, but instead goes through off-site screening facilities first.
The letter sent to Bloomberg's group Mayors Against Illegal Guns was opened by its director.
Authorities have intercepted several ricin-laced letters in recent weeks. The poisonous substance is found naturally in castor beans but can be converted into a lethal form that can cause death within days. There is no known antidote.
The FBI is moving cautiously after wrongly arresting a suspect in another recent ricin case, the source said.
James Everett Dutschke, a Tupelo, Mississippi martial arts instructor, was arrested in April with allegedly sending poison-laced letters to Obama and two other public officials after authorities first arrested another man who worked as an Elvis impersonator. (Additional reporting Susan Heavey in Washington, Edith Honan and Chris Francescani in New York and Eric Johnson in Seattle; Editing by Ellen Wulfhorst, Sofina Mirza-Reid and Tim Dobbyn)