The first of the three recent ricin investigations in the U.S. began in April.
An affidavit says authorities found a dust mask and other items that tested positive for ricin.
An FBI agent testified that no evidence of ricin was found in searches of Curtis' home.
Senate Majority Leader Harry Reid of Nevada said Tuesday that the toxin might be ricin.
In Washington, a letter sent to President Barack Obama initially tested positive for the poison ricin.
Holland told CNN the letter originally tested negative for ricin but was being retested Wednesday.
Investigators were testing the contents of the letter to determine whether ricin was inside, he said.
Holland is the son of Lee County Judge Sadie Holland, who received one of the ricin-laced letters.
CNN: Suspect in ricin case described as 'a little different'
The Democratic incumbent whom he challenged was the son of the judge who received a ricin-laced letter.
Roger Wicker says he once hired the man accused of mailing ricin-filled letters as an Elvis impersonator.
"Over the course of years we've had some situations where there have been ricin scares, " Donahoe said.
He declined to comment on the significance of the preliminary ricin result, referring questions to the FBI.
According to the Centers for Disease Control and Prevention, ricin is a poison found naturally in castor beans.
Browne would not say whether the letters were believed to be linked to any other recent ricin cases.
The federal government has offered no evidence he had ricin or castor beans in his possession, she said.
Roger Wicker, a Mississippi Republican, received a letter containing ricin, a toxin derived from the castor oil plant.
Cohen said it appears the ricin in all three cases was a crude form that's relatively easy to make.
The FBI has the lead for determining whether a suspicious powder is a dangerous substance, such as a ricin.
Cohen said ricin is not common because other poisons, such as anti-freeze, can easily be bought at a store.
The suspect in the ricin case, Paul Kevin Curtis, was arrested at his home in Corinth, Mississippi, on Wednesday.
Ricin interferes with the process of protein synthesis, eventually leading to cell death.
The FBI has not yet revealed details about how lethal the ricin was.
Veale declined to comment on whether the substance was believed to be ricin.
Other ricin incidents have been unrelated to any specific ideology, the ADL said.
Both sources told CNN that the wife told authorities she found a suspicious container and ricin research on their computer.
There haven't been any more ricin-type suspicious letters to lawmakers since the mailing to Wicker, Capitol Police in Washington said.
Several emergency services staff had been taken to hospital to be screened for possible ricin contamination, but were later discharged.
Three subsequent tests of the mask by the National Bioforensic Analysis Center came back positive for ricin, the document states.
The FBI has not yet revealed details about how the ricin was made or how lethal it may have been.
Officials previously revealed that two threatening letters postmarked in Louisiana and sent to Bloomberg contained traces of the deadly poison ricin.
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