U.S. officials say a letter that preliminarily tested positive for ricin, a potentially fatal poison, has been sent to the office of a senator.
The letter was addressed to Senator Roger Wicker, a Republican from Mississippi.
Officials said the letter had been postmarked from Memphis, Tennessee.
No injuries have been reported as a result of the sending.
Officials said the the presence of ricin was detected in preliminary tests at a mail screening facility outside Senate buildings.
Further tests on the substance are being carried out by an accredited laboratory.
Experts say ricin must be ingested by humans to be lethal.
Mail to the U.S. Congress has been screened off-site since letters laced with anthrax were sent to lawmakers in 2001, after the September 11 hijacked airliner attacks.
Based on reports from Reuters, AP and AFP