One senator, Claire McCaskill of Missouri, said authorities have a suspect in the fast-moving case, but she did not say if an arrest had been made. She added the letter was from an individual who frequently writes lawmakers.
The FBI and U.S. Capitol Police are both investigating.
Terrance W. Gainer, the Senate sergeant-at-arms, said in an emailed message to Senate offices that the envelope to Wicker had no obviously suspicious outside markings, bore a postmark of Memphis, Tenn., and lacked a return address.
He added there was "no indication that there are other suspect mailings," but urged caution.
The letter was discovered at a mail processing plant in Prince George's County in suburban Maryland, said Sen. Dick Durbin, D-Ill.
Wicker's office issued a statement saying "any inquiries regarding member security must be directed to the United States Capitol Police."
Capitol Police had no immediate comment.
But Majority Leader Harry Reid told reporters of the letter, and other lawmakers said they had been provided information by the office of the Senate sergeant-at-arms.
Milt Leitenberg, a University of Maryland bioterrorism expert, said ricin is a poison derived from the same bean that makes castor oil. He said it must be ingested to be fatal.
"Luckily, this was discovered at the processing center off premises," Durbin said. He said all mail to senators is "roasted, toasted, sliced and opened" before it ever gets to them.
One law enforcement official said evidence of ricin appeared on preliminary field tests of the letter, although such results are not deemed conclusive without further testing. The official spoke on condition of anonymity because the investigation remains active.
The discovery evoked memories of the days after the terrorist attacks of Sept. 11, 2001, when mail laced with anthrax began appearing in post offices, newsrooms and congressional offices.
That included letters sent to Sen. Tom Daschle, D-S.D., who was Senate majority leader, and Sen. Patrick Leahy, D-Vt. Two Senate office buildings were closed during that investigation.
Overall, five people died and 17 others became ill. The FBI attributed the attack to a government scientist who committed suicide in 2008.
More immediately, though, the discovery came as lawmakers were demanding answers to the attacks in Boston a day earlier.
There was no evidence of a connection between the bombings and the letter addressed to Wicker, a Mississippi Republican.
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