Sen. Carl Levin, D-Mich., said it was clear the revised mail screening system had worked as it should.
"It was caught in a way that did no damage, so I'm not obsessively concerned," Levin said.
Sen. Jeff Flake called the letter "a big concern, obviously, for all of us." The Arizona Republican described senators as "very anxious to get more details."
Wicker was appointed to the Senate in 2007 before winning a full term last year. He previously was a House member for 12 years. He is viewed as a solid conservative.
Milt Leitenberg, a University of Maryland bioterrorism expert, said ricin is a poison derived from the same bean that makes castor oil. According to a Homeland Security Department handbook, ricin is deadliest when inhaled. It is not contagious, but there is no known antidote.
Associated Press writers Donna Cassata, Andrew Miga, Seth Borenstein, Eric Tucker, Eileen Sullivan, Pete Yost and Pauline Jelinek in Washington and Emily Wagster Pettus and Jeff Amy in Jackson, Miss., contributed to this report.
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