An envelope addressed to a US senator has tested positive for ricin, a potentially fatal poison, congressional officials have said.
The letter to Senator Roger Wicker of Mississippi was discovered on Tuesday at a mail processing plant in Prince George's County in suburban Maryland, said Democratic Senator Dick Durbin.
"Luckily, this was discovered at the processing centre off premises," Durbin said, adding that all mail to senators are thoroughly scanned and opened before being delivered.
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. Other legislators said that they were informed of the incident by the office of the Senate Sergeant-at-Arms.
The FBI, the Capitol Police and other agencies are involved in the investigation, the sergeant said in an email to Senate employees.
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.
The evidence of ricin appeared on preliminary field tests of the letter, although such results are not deemed conclusive without further testing, said a law enforcement official on condition of anonymity.
In September 2001, days after the attack on the World Trade Centre and Pentagon, mail laced with anthrax appeared in post offices, newsrooms and congressional offices, killing five and infecting 17.
The FBI attributed the anthrax attacks to a government scientist who committed suicide in 2008.
The ricin-laced letter comes a day after two explosions went off at the Boston Marathon, killing three people and injuring 176.