WASHINGTON — An envelope addressed to Senator Roger Wicker of Mississippi tested positive Tuesday for ricin, a potentially fatal poison, congressional officials said, heightening concerns about terrorism a day after a bombing killed three and injured more than 170 at the Boston Marathon.
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 US Capitol Police are both investigating.
Terrance W. Gainer, the Senate sergeant-at-arms, said in an e-mailed message to Senate offices that the envelope to Wicker, a Republican, had no obviously suspicious outside markings, bore a postmark of Memphis, 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 Senator Dick Durbin, Democrat of Illinois.
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.
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.