WASHINGTON -- An envelope addressed to a Republican senator twice tested positive Tuesday for ricin, a potentially fatal poison, congressional officials said, heightening concerns about terrorism a day after a bombing killed three and left more than 170 injured 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 sent from an individual who frequently writes lawmakers.

Late Tuesday, Sen. Roger Wicker of Mississippi released a statement acknowledging the letter and said it was sent to his Washington office.

"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," Wicker said. "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."

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.

The U.S. Capitol Police, which is also investigating the case, declined to comment.

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 and lacked a return address. It bore a postmark from Memphis, Tennessee.

Mail from a broad swath of northern Mississippi, including the Memphis suburbs of DeSoto County, Mississippi, Tupelo, Oxford and the northern part of the Mississippi Delta region, is processed and postmarked in Memphis, according to a Postal Service map. The Memphis centre also processes mail for residents of western parts of Tennessee and eastern Arkansas.

Gainer said 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, the No. 2 Senate Democrat.

Wicker, 61, 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 of Representatives.

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.

Majority Leader Harry Reid told reporters of the letter. Other lawmakers said they had been provided information by Gainer's office.

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 centre 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 two Democrats -- Sen. Tom Daschle, who was Senate majority leader at the time, and Sen. Patrick Leahy. 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.

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Associated Press writers Andrew Miga, Seth Borenstein, Eric Tucker, Henry C. Jackson and Eileen Sullivan in Washington and Emily Wagster Pettus and Jeff Amy in Jackson, Mississippi, contributed to this report.