WASHINGTON (WUSA9) -- Just a day after the deadly terror attack in Boston, a scare in the nation's capital.

An envelope believed to contain the deadly poison ricin was found at an off-site mail facility that screens congressional mail.

It contained a white substance. The letter was addressed to Republican Mississippi Senator Roger Wicker.

In an evening briefing, on Capitol Hill about the Boston bombings, Senators were also told about the suspicious letter. Postal workers noticed the white granular substance at its Senate mail handling facility in Hyattsville.

John Miller, Senior CBS Correspondent said, "The system worked the way it was suppose to. It was identified as suspicious. They did a field test that came back positive for ricin. They did a second field test to confirm that.But those field tests are indicators. So what they've done is send it to Baltimore to a lab to grow it out to find out if in fact it was ricin."

The letter never made it to the Capitol. It's customary for mail to be screened off-site ever since the anthrax scares of 2001 and 2004 when anthrax-laced letters reached the Capitol building.

The U.S Postal Service says they have not found other suspicious mail.Sen. Claire McCaskill says authorities have a suspect in the case, but there's been no word of an arrest. McCaskill says the letter was from an individual who frequently writes to lawmakers.

Authorities do not know if there is any connection to the terror attack in Boston.Some lawmakers say it's unlikely there is a link.

Rep. Michael McCaul, (R) Texas said, "It did test positive for ricin, we have no evidence whatsoever connecting the two, between the ricin incident and the Boston bombing incident. Since 9/11, since the anthrax scare, we have an offsite mail facility, and it actually worked in this case, where it was caught in an advanced screen and prevented from getting to Capitol Hill in the offices. So the system worked. Ricin is very targeted, almost assassin like lethal dose. It's not intended to inflict mass casualties. The terrorists are more interested in killing, in having mass casualties so it would be inconsistent with the terrorist philosophy."

Senator Wicker released this statement this evening acknowledging the suspicious letter.

"This matter is part of an ongoing investigation by the United States Capitol Police and FBI. I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. Gayle and I appreciate everyone's thoughts and prayers."

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