Capitol Hill police say a letter mailed to the office of Sen. Roger Wicker, R-Miss., tested positive for a dangerous poison called ricin Tuesday.
"The envelope was immediately quarantined by the facility's personnel and USCP HAZMAT responded to the scene," Capitol Police spokesman Shennell S. Antrobus said.
"Preliminary tests indicate the substance found was ricin," she continued. "The material is being forwarded to an accredited laboratory for further analysis."
Sen. Claire McCaskill, D-Mo., says the letter was sent from someone who often writes to members of Congress.
There was no evidence of a connection to the Boston bombings, but the news dredged up memories of the anthrax scare that followed the attacks of 9/11.
Meanwhile, Terrance Gainer, the Senate's sergeant-at-arms, told Senate offices there is "no indication that there are other suspect mailings."
Nevertheless, the mail facility in question will be closed for several days pending an investigation.
"Our primary concern right now is the safety of our employees, the safety of our customers and the safety of the U.S. mail," Postal Service spokeswoman Patricia Licata said.