More to follow…

Via Politico:

An envelope sent to an office of Sen. Roger Wicker (R-Miss.) included a substance that has tested positive for Ricin, two sources say.

It was not immediately clear when the envelope was received or whether it was sent to his Washington, D.C., office or a field office.

Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller are briefing senators now.

Update: Sounds like they know the person.

[T]he FBI had no comment on the matter and the Secret Service said they’re not involved in the investigation, if there is one.

Sen. Claire McCaskill (D-Mo.) said members were briefed that the substance had been found in a letter and a suspect has been identified.

McCaskill said the letter came from an individual who frequently writes to lawmakers. She wouldn’t identify the person but confirmed officials had identified someone.

McCaskill said state offices have been told what to look for if there are more letters containing the toxic substance.