My instinct whenever there’s a story like this is “false positive,” but apparently not: CNN claims the letter’s been tested three times and every time has been a red alert. Also, would DHS go to this trouble if they had any doubt?

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

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

A personal briefing by the head of Homeland Security and the director of the FBI for a possible false positive?

CNN says the letter never made it to the Hill. It was caught at an off-site mail facility, tested, re-tested, re-re-tested and then sent to a lab in Maryland. But there’s probably more out there, just because Wicker’s one of the most random low-profile members of the Senate. Unless someone’s running a vendetta against him for reasons we don’t know, there’s no reason to single him out. He must have been targeted because he’s part of something — part of the Senate/Congress, part of the GOP, maybe part of some coalition on a particular vote — and that means the other parts are in danger too. Stand by for updates.

Update: More from Politico. Very strange — maybe a vendetta after all, and to all appearances not linked to Boston.

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.

Turns out Napolitano and Mueller were already set to appear before the Senate to address cybersecurity and that the ricin letter “came up” at the hearing, so it’s not as urgent as it first appeared.

Update: And, thankfully, the story may be falling apart:

CNN guest says he has intel that 2 of the 3 tests for ricin came back negative/inconclusive.

— Josh Kraushaar (@HotlineJosh) April 16, 2013