(ABC News) -- A suspicious letter, potentially laced with poison, ricin, was sent to the office of Sen. Roger Wicker of Mississippi, ABC News has learned.
Senate Majority Leader Harry Reid confirmed to ABC News that Capitol offices were on high alert. He referred questions to Capitol Police.
Two sources confirmed the letter was sent to Wicker, R-Miss., but did not arrive at his office on Capitol Hill. It was stopped at a mail processing facility.
Wicker's Dirksen Capitol Hill office is closed for the evening, as it is after office hours.
Aides in Wicker's office emphasized that at no point did the senator's office evacuate or close because of the threat.
"Once we get information from Capitol Hill police, we will send out more," an aide to Wicker said Tuesday evening.
Sen. Jeff Flake, R-Ariz., said all mail delivery has been stopped as a precaution.
"We are very concerned," he said.
Homeland Security Secretary Janet Napolitano, on Capitol Hill, declined to comment tonight on the suspicious letter.
ABC News' Pierre Thomas and Jack Date contributed to this report.