WASHINGTON — An envelope addressed to Sen. Roger Wicker of Mississippi twice tested positive Tuesday for ricin, a potentially fatal poison, congressional officials said, heightening concerns about terrorism a day after a bombing killed three at the Boston Marathon.
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.
On Tuesday night, FBI spokesman Paul Bresson said initial field tests on the substance produced mixed results and that it is in the process of undergoing further analysis at an accredited laboratory. Only after that testing can a determination be made about whether the substance is ricin, Bresson said.
Late Tuesday, Wicker, a Republican, released a statement acknowledging the letter and said it was sent to his Washington office.
"I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe," he said.
Terrance 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 and lacked a return address. It bore a postmark from Memphis.
Mail from a broad swath of northern Mississippi, including Tupelo and Oxford, is processed and postmarked in Memphis, according to a Postal Service map. The Memphis center also processes mail for residents of western parts of Tennessee and eastern Arkansas.
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, 61, was appointed to the Senate in 2007 and won election to a full term last year. He previously served a dozen years in the House. He has a solidly conservative voting record.
The letter 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.