A National Guard Civil Support team, whose primary role is identifying weapons of mass destruction hazards, has been deployed to Spokane to investigate the discovery of several threatening letters that have preliminarily tested positive for Ricin.
Ricin is a highly toxic substance made from castor beans. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult.
There is no known antidote.
The 10th Civil Support Team, a full-time National Guard unit whose role is to identify and assess weapons of mass destruction, has deployed 20 personnel and nine vehicles to Spokane to investigate the letters found at the downtown post office earlier this week.
The Guardsmen are responding to the discovery of those two letters, which the American Postal Workers Union alerted employees about on Wednesday.
According to the APWU website, two letters which preliminarily tested positive for Ricin and addressed to the downtown Spokane Post Office and a federal judge, arrived at the Spokane Post Office on May 14. The letters had been postmarked on May 13.
The releases on the union's website indicated that "they have no reason to believe that any employees are at risk from handling the suspect letters as they passed through the mailstream in Spokane."
The FBI confirmed Thursday afternoon that it was assisting in identifying the source of the letters.
The discovery of Ricin in Spokane comes less than a month after a Tupelo, Miss., man was arrested for possession and use of a biological agent as a weapon in connection with letters sent to President Barack Obama, Sen. Roger Wicker, R-Mississippi, and a local judge in Lee County, Miss.
Those letters arrived at post offices on April 16 and created some anxiety in Washington, D.C., in the wake of the Boston Marathon bombings one day before April 15.