By Susan Cornwell and Marilyn W. Thompson
WASHINGTON (Reuters) - The FBI said on Thursday that more tests may be necessary to determine the potency of a granular material identified as ricin that was packed into letters sent to President Obama, a U.S. senator and a Mississippi judge.
Last week, the FBI said laboratory testing had confirmed the presence of the toxin in the letters. But FBI spokesman Chris Allen said Thursday he was reluctant to say all tests were complete because "they may be doing more tests on it as the investigation is ongoing."
The Centers for Disease Control said Thursday it had finished its evaluation of the material and sent the results to the FBI and other agencies. She said the CDC could not discuss its analysis because the investigation is continuing.
In Mississippi, the FBI continued its pursuit of the case after last week releasing its original suspect, a man who was arrested after preliminary field tests of the letters detected the chemical agent.
"I can confirm that the CDC did receive samples of material for testing. We have completed the tests and reported the results to partners, including the FBI," CDC spokeswoman Barbara Reynolds said.
Ricin, which is made from castor beans, is a poison that can be deadly to humans and is considered a potential terror weapon, particularly if refined into an aerosol form.
Letters addressed to Senator Roger Wicker, a Mississippi Republican, and Democratic President Barack Obama were retrieved last week at off-site mail facilities before reaching their intended victims. But the discovery added another layer of anxiety as authorities dealt with bombings at the Boston Marathon.
After preliminary tests of the letters proved positive for ricin, the material was sent to the CDC, which houses labs used to analyze suspicious materials in cases deemed a possible public health threat.
An FBI agent testified in court in Mississippi that the ricin found in the letters was in a crude form and looked like castor beans ground up in a blender, according to press accounts. Experts have said ricin in this form would have a low potency.
U.S. prosecutors on Tuesday dropped charges against Paul Kevin Curtis, a 45-year-old Elvis impersonator they had accused of sending the ricin-laced letters. The case was dismissed "without prejudice," meaning the charges could be potentially reinstated if warranted.
Authorities searched the house and business address of a second Mississippi man, Tupelo martial arts instructor Everett Dutschke, but he has not been charged in the ricin case. On Thursday, agents searched a private residence where a friend of Dutschke's lives. The friend is not a suspect in the case, according to the Northeastern Mississippi Daily Journal.
Milton Leitenberg, senior research scholar at the Center for Strategic and International Studies at the University of Maryland, said that the vast majority of ricin cases since the 1960s have involved crude ricin preparations made from recipes published in manuals and on the Internet.
"You could ingest this crude stuff, swallow a couple of tablespoons and you'd probably vomit, but not much more," Leitenberg said in a telephone interview.
To do more harm, he said, "you have to make a good material, and then you have to get it someplace." A material like that described in the ricin court hearing would pose little danger, Leitenberg said.
In an earlier case, CDC researchers conducted a detailed analysis of ricin found in February 2008 in a Las Vegas hotel room. They found it to be in a primitive form - apparently concocted with castor beans from a recipe in a publication called "The Anarchist Cookbook."
Another expert who was at the CDC during the 2001 anthrax letter attacks that killed five people in the United States, said it took a lot of skill to purify ricin and make it into a serious threat.
"There is a difference between being about to build it and disseminating it and making someone sick. It truly looks like someone has done an initial stage," said Sean Kaufman, now at Emory University.
(Reporting By Susan Cornwell; Additional reporting by Julie Steenhuysen in Chicago; Editing by Stacey Joyce)