Sen. Roger Wicker, R-Miss. (AP/J. Scott Applewhite)

A letter was mailed to Mississippi Republican Sen. Roger Wicker's office containing the toxic substance ricin, forcing the temporary closure of a Senate post office and prompting a federal investigation.

The letter was intercepted at an off-site location on Tuesday morning and was later tested positive for ricin, a highly poisonous chemical that can be fatal to humans if inhaled. While the letter did not reach Wicker's office, the discovery prompted the off-site center to be shut down. There is no evidence of contamination in the on-site post office.

Wicker's office released a statement on Tuesday evening from the senator in which he did not specifically mention the letter or its contents. He said the matter was "part of an ongoing investigation" by the U.S. Capitol police and the 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 [my wife] and I appreciate everyone’s thoughts and prayers," Wicker said.

Also Tuesday evening, the Senate sergeant-at-arms, Terrance Gainer, sent an email to senior Senate staff saying the letter had been postmarked from Memphis, Tenn., but had no unusual identifying marks. He urged staffers to be "vigilant in their mail handling processes for ALL mailings."

Senators first learned of the letter at a private security briefing about the Boston Marathon bombings, but there is no indication, senators said, that the poisonous letter was connected to Monday's attacks.

"We don't know enough about Boston to even speculate," said Illinois Democratic Sen. Dick Durbin after the briefing.

Missouri Democratic Sen. Claire McCaskill said the sender was a person who mailed frequent letters to Capitol Hill. Lawmakers' state offices around the country are currently being notified about the incident.

All mail to the Capitol building is processed and tested in a facility off-site before reaching the destination as a precaution against incidents like this. Several senators interviewed said they were not concerned about a broader attempt to target them.

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"It was caught in a way that did no damage, so I'm not excessively concerned about it," said Michigan Democratic Sen. Carl Levin. "They've taken strong steps to avoid any problem."

Letters containing the lethal substance anthrax were sent to Vermont Democratic Sen. Pat Leahy and then-Senate Majority Leader Tom Daschle exactly one week after the Sept. 11, 2001, terror attacks. Several news organizations were sent anthrax-laced mail as well. Five people were killed and 17 were injured after inhaling the substance.

Beth Fouhy and Olivier Knox contributed to this report.