As part of a tense day in Washington, the FBI reported Wednesday around midday that a second ricin-contaminated envelope sent to a mail processing facility in Washington, D.C. Its intended recipient appears to have been the president. At another point, two Senate office buildings were locked down, one due to a person who apparently was carrying suspicious envelopes. (Update: Capitol police say the letters found today weren't hazardous, but a man was still being questioned late Wednesday, according to the AP. Update No. 2: There's been an arrest made. More here.)

Fox News' Mike Levine reports on an FBI bulletin that identified the envelope intended to reach the president.

Sky News specifies that the originating city was Memphis.

ABC News reports that the Secret Service has confirmed the discovery.

In a statement this afternoon, the FBI denied a link to Boston, which seems clear given the postmark date.

According to Fox News, the FBI released the contents of the letters.

The bulletin said both letters included the phrase: "To see a wrong and not expose it, is to become a silent partner to its continuance." 

Both were signed, "I am KC and I approve this message." 

During his daily briefing, White House spokesman Jay Carney offered little new information, saying only that Obama had been briefed on the letters both last night and this morning.

It's not clear why Sen. Roger Wicker, the junior senator from of Mississippi, was one of the targets, as first reported Tuesday night. He was one of the few Republicans to support moving forward on a package of gun legislation, but that vote occurred later in the week than the letters were apparently sent.

Contrary to reports that indicated a sole sender who was in custody following the letter sent to Wicker, Levine also indicates that the FBI is "pursuing multiple leads" and that the terrorism task force is involved. NBC News indicates that the FBI "believe they know who sent the letters, but no arrest [has been] made."

According to Fox News, the bulletin indicates that testing so far has been "inconsistent," indicating that the presence of the castor-bean-derived poison is not settled.

ABC's Terry Moran reported that the letter was received at the White House screening facility, unlike the letter sent to Wicker. That reduces the possibility that the positive test was the result of cross-contamination, a problem that arose during the post-9/11 anthrax scare.

There was tension elsewhere in the capital. Shortly after noon, the Hart Office Building on Capitol Hill was closed due to what CNN described as an individual wearing a backpack containing other letters. In a brief press conference, the Capitol Police's Shennell Antrobus indicated that suspicious envelopes have been found in both the Russell and Hart office buildings. "All I can tell you right now," he said, "is we're investigating suspicious envelopes in both."