One senator, Claire McCaskill of Missouri, said authorities have a suspect in the fast-moving ricin 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. Both declined to comment.
Terrance W. Gainer, the Senate sergeant-at-arms, said in an emailed message to Senate offices that the envelope to Wicker, a Republican, had no obviously suspicious outside markings and lacked a return address. It bore a postmark from Memphis, Tenn., where mail from parts of northern Mississippi has long been processed.
He added there was "no indication that there are other suspect mailings." Yet he urged caution, and also said the Senate off-site mail facility where the initial tests were performed on the letter will be closed for a few days while the investigation continues.
The letter was discovered at a mail processing plant in Prince George's County in suburban Maryland, according to 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."
The 61-year-old lawmaker was appointed to the Senate in 2007 and won election to a full term two years ago. He previously served a dozen years in the House.
He has a solidly conservative voting record, so much so that he drew notice last week when he voted to allow debate to begin on controversial gun legislation in the Senate. "I cast this vote at the request of the National Rifle Association, of which I am a member," he said in a statement at the time that added he has a 100 percent voting record in favor of Second Amendment rights.
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. According to a Homeland Security Department handbook, ricin is deadliest when inhaled. It is not contagious, but there is no antidote.
"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 two 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.
Associated Press writers Andrew Miga, Seth Borenstein, Eric Tucker, Henry Jackson and Eileen Sullivan in Washington and Emily Wagster and Jeff Amy in Jackson, Miss., contributed to this report.