An envelope that tested positive for the deadly poison ricin was intercepted Tuesday at the U.S. Capitol's mail facility, according to news reports.
Senate Majority Leader Harry Reid (D-NV) said he was told the letter was addressed to Sen. Roger Wicker (R-MS) before being intercepted.
A Maryland lab confirmed the presence of ricin after initial field tests indicated the poison was present, according to Senate Sergeant-at-Arms Terrance Gainer.
The FBI cautioned that more comprehensive testing is still necessary.
"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," according to the bureau.
"Those tests are being conducted and generally take from 24-48 hours."
In a statement Tuesday, the U.S. Capitol Police said more tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland.
The letter had a Memphis, Tennessee, postmark and no return address.
Sen. Claire McCaskill (D-MO) told reporters that a suspect has already been identified in the incident, but other sources said no one was in custody last night.
Wicker has been assigned a protective detail for the time being.
Postal workers started handling the mail at a site just outside Capitol Hill after the infamous 2001 anthrax attacks that targeted a number of U.S. Senators.
The post office will be temporarily shut down "to make sure they get everything squared away," McCaskill said Tuesday, reiterating that all is fine.
"The bottom line is, the process we have in place worked," she said. "Members will be warning their home-state offices to look out for similar letters."
A previous ricin scare hit the Capitol in 2004, when tests identified a letter in a Senate mailroom that served then-Majority Leader Bill Frist's office.
The discovery forced 16 employees to go through decontamination procedures, but no one reported any ill effects afterward, Frist said at the time.
Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms - an amount the size of the head of a pin - can kill an adult.
There is no specific test for exposure and no antidote once exposed.
Ricin can be produced relatively easily and cheaply, and authorities in several countries have investigated links between suspect extremists and ricin.
Experts say it is more effective on individuals than on mass groups.
No known link has been established between the ricin envelope and the Boston Marathon bombings Monday, though officials are still investigating both.