Senate Offices Evacuated Over Series of Suspicious Packages

U.S. Capitol Police on Wednesday evacuated the first floor of the Hart Senate Office Building due to a suspicious package. People on other floors of the building were ordered to go into their offices.

A suspicious envelope was also found at the office of Alabama Senator Richard Shelby in the Russell Senate Office Building. Security cleared the hall but there was not an official evacuating.

Related: Working to Re-Define 'Forensic Intelligence'

The moves came after U.S. Secret Service said that it intercepted a letter addressed to President Barack Obama that contained a "suspicious substance."

Secret Service spokesman Ed Donovan said the letter was intercepted on Tuesday at a facility away from the White House.

The announcement comes a day after lawmakers said a letter was mailed to Senator Roger Wicker, a Mississippi Republican, that tested positive for the lethal poison ricin. That letter was also intercepted at an off-site mail screening facility.

It tested positive for ricin in a field test, but the FBI said on Wednesday that tests have shown "inconsistent results" and the substance is being further analyzed.

That envelope had a Tennessee postmark and no return address. It contained the chilling note: "You haven’t listen to me before. Now you will, even if people have to die."

The authorities have not yet linked the incidents to Monday’s terrorist attack in Boston, but it has further heightened security concerns.

Officials would not be drawn on a possible suspect, but Sen. Claire McCaskill told the Associated Press that police have someone in mind. "The person that is a suspect writes a lot of letters to members," she said.

Senate Republican leader Mitch McConnell said on Wednesday: "Monday's attack in Boston reminded us that terrorism can still strike anywhere at any time. And as yesterday's news of an attempt to send ricin to the Capitol reminds us, it is as important as ever to take the steps necessary to protect Americans from those who would do us harm."

A FBI spokesman said: "Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests are in the process of being conducted and generally take from 24 to 48 hours."

The Capitol was hit by a ricin scare in 2004 when tests identified a letter in a Senate mailroom for then-Majority Leader Bill Frist's office.

The discovery meant 16 employees had to go through decontamination procedures, though no one reported any ill effects afterward, CNN reports.

Ricin is a highly toxic substance derived from castor beans. Just 500 micrograms could kill an adult. There is no specific test for exposure and no antidote once exposed.

It can be produced easily and cheaply and experts say it is more effective on individuals than as a mass chemical weapon .

Bulgarian dissident Georgi Markov was killed by ricin nine years after he defected to London. He was jabbed by the tip of an umbrella while waiting for a bus in London and died four days later.

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