Roger Wicker, a Mississippi Republican, received a letter containing ricin, a toxin derived from the castor oil plant.
Three of four field tests conducted on the powder inside the envelope addressed to Wicker tested positive for a protein that later tests determined to be ricin, a lethal toxin, it says.
Thomas Pittman, a retired professor at the University of Southern Mississippi and an expert in toxicology, said ricin has a number of qualities that make it desirable to someone bent on doing harm: The ingredients are relatively easy to obtain, it's not that hard to make a crude form of it, it's deadly and it's hard to detect in a victim's body.
Meanwhile, in what appears to be a separate incident of possibly poisoned mailings in Washington state, the FBI confirmed the presence of ricin on a letter sent to Fairchild Air Force Base near Spokane, the agency said.
According to the Centers for Disease Control and Prevention, ricin is a poison found naturally in castor beans.
Dutschke, the affidavit stated, also said he'd never researched anything about ricin, a toxin derived from castor beans that has no known antidote.
The Democratic incumbent whom he challenged was the son of the judge who received a ricin-laced letter.
The FBI has the lead for determining whether a suspicious powder is a dangerous substance, such as a ricin.
The FBI said the substance tested positive for ricin, a toxin derived from castor beans that has no known antidote.
The FBI said the letters tested positive for ricin, a toxin derived from castor beans that has no known antidote.
Dr Vitetta's team genetically engineered three versions of ricin's A chain.
Authorities say Curtis sent a letter that may have contained ricin to Sadie Holland, a judge who sentenced him to six months in jail in an assault case a decade ago.
In 2008, authorities said a man in Las Vegas may have accidentally poisoned himself with ricin that he had made from a backyard castor plant.
However, using the seeds to make a highly concentrated form of ricin would require laboratory equipment and expertise to extract, said Raymond Zilinskas, a chemical and biological weapons expert.
Security and counterterrorism expert Michael Fagel, who teaches at Northwestern University and is a veteran of ricin investigations, said ricin may be employed because castor beans are so easy to come by.
Cohen said it appears the ricin in all three cases was a crude form that's relatively easy to make.
Both sources told CNN that the wife told authorities she found a suspicious container and ricin research on their computer.
In Washington state, a 37-year-old was charged last week with threatening to kill a federal judge in a letter that contained ricin.
"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin, " according to the bureau.
The most notable case of ricin poisoning was in 1978, when a Bulgarian dissident was lethally injected with it by an operative of that country's secret service.
The men were arrested on Tuesday days after a laboratory test found trace amounts of ricin in their possession, the authorities said.
But according to the affidavit, Dutschke insisted that he'd never researched anything about ricin and that he'd never even seen a castor bean.
In 2007, Dutschke failed in his bid as a Republican to defeat Democratic state representative Steve Holland, whose mother, a judge, received one of the ricin-tainted letters.
An affidavit says authorities found a dust mask and other items that tested positive for ricin.
Preliminary tests on filters at a government mail-screening facility also indicated the presence of ricin Wednesday morning, and mail from that site also was being tested, the FBI said.
After the arrest, he consented to the seizure of his laptop computer, a hard drive and several flash drives, the affidavit in the ricin case states.
But members of the New York Police Department emergency service unit who came in contact with the letter that was opened at a city mail facility were examined for "minor symptoms" of ricin exposure that they experienced on Saturday, according to Paul Browne, NYPD spokesman.
In Washington, a letter sent to President Barack Obama initially tested positive for the poison ricin.
Cohen said ricin is not common because other poisons, such as anti-freeze, can easily be bought at a store.
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