the letter was tested positive for ricin .
story highlights laboratory tests confirmed the presence of ricin , senate official says the letter had a memphis , tennessee , postmark and no return address it was addressed to sen. roger wicker , r - mississippi ricin is a highly toxic substance derived from castor beans an envelope that tested positive for the deadly poison ricin was intercepted tuesday afternoon at the u.s. capitol 's off - site mail facility in washington , congressional and law enforcement sources tell cnn .
” ( photos : roger wicker 's career ) the capitol police says it is partnering with the fbi for an “ ongoing investigation .
a laboratory in maryland confirmed the presence of ricin after initial field tests indicated the poison was present , according to senate sergeant - at - arms terrance gainer .
the senate mail facility on tuesday notified the u.s. capitol police that it had received an envelope containing “ a white granular substance , ” according to a release .
the letter was postmarked memphis , tenn. , and had no suspicious markings or return address , according to a message sent to congressional staffers by senate sergeant at arms terrance gainer .
senate majority leader harry reid said he was told the letter was addressed to the office of sen. roger wicker , r - mississippi .