An envelope with traces of the highly toxic
poison ricin was quarantined by the US Senate post office responsible
for checking all mail for dangerous substances, according to media
reports late Tuesday.
The letter had been addressed to Republican Senator Roger Wicker of Mississippi, according to Politico newspaper online.
"Preliminary tests indicate the substance found was ricin," according to a statement from the US Capitol Police.
Officials said that there appeared to be no connection to the terrorist attack Monday in Boston.
After anthrax attacks were delivered by mail to media outlets and elected officials in 2001, killing five people and injuring 17 others, the post office routinely screens all letters going into Congress.
The Senate screening facility was shut down during the investigation by the Capitol Police and FBI, and Senate mail will be screened for the time being in the House of Representatives screening site, officials said.
Wicker thanked law enforcement officials for their "diligence' in keeping those who work in the Capitol safe.
Ricin is produced from the castor oil plant, and a small amount can be lethal, according to the Centers for Disease Control in Atlanta, Georgia.
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