A letter was mailed to Mississippi Republican Sen. Roger Wicker's office containing the toxic substance ricin, Senate Majority Leader Harry Reid confirmed to reporters on Tuesday.
The letter was intercepted at an off-site location Tuesday morning and was later tested positive for ricin, a highly poisonous chemical that can be fatal to humans if inhaled.
While the letter did not reach Wicker's office, the discovery prompted the Senate post office to be shut down. There is no evidence of contamination in the on-site post office.
Senators learned of the discovery at a private security briefing about the Boston Marathon bombings, but there is no indication, senators said, that the poisonous letter was connected to Monday's attacks.
"We don't know enough about Boston to even speculate," said Illinois Democratic Sen. Dick Durbin after the briefing.
Missouri Democratic Sen. Claire McCaskill said the sender was a person who mailed frequent letters to Capitol Hill. Lawmakers' state offices around the country currently being notified about the incident.
All mail to the capitol building is processed and tested in a facility off-site before reaching the destination as a precaution against incidents like this. Several senators interviewed said they were not concerned about a broader attempt to target them.