Update at 8:44 p.m. ET. Authorities Make An Arrest:
Authorities have made an arrest in connection to the suspicious envelopes sent to a senator and President Obama.
The FBI identified the suspect as Paul Kevin Curtis.
"The letter, received by [Sen. Roger Wicker's] office on Tuesday, had a Memphis postmark and no return address, Senate Sergeant at Arms Terrance Gainer wrote in an e-mail to senators and aides. Tupelo's mail is routed through Memphis.
"The ricin was confirmed by preliminary tests on Tuesday. On Wednesday, Obama received a similar letter, and once again, preliminary tests showed there was ricin in it."
The AP reports that the letters sent to Obama and Wicker were similar. They read, according to the AP: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both of them were signed, "I am KC and I approve this message."
Update at 8:28 p.m. ET. Three Letters:
In a statement, the FBI says authorities arrested Paul Kevin Curtis. That name is different from the name that the Clarion Ledger and other news organizations were reporting. We've updated this post to reflect that.
The FBI said Curtis is "believed to be responsible for the mailings of the three letters sent through the U.S. Postal Inspection Service which contained a granular substance that preliminarily tested positive for ricin. The letters were addressed to a U.S. Senator, the White House and a Mississippi justice official."
FBI agents made the arrest in Corinth, Miss.
Update at 8:15 p.m. ET. Ricin Was Low-Grade:
CBS News reports that "when the results came back from the Ft. Dietrick lab Wednesday, various officials have described them as 'weak,' 'low-grade' and 'less than one percent toxin.'"
The AP, which is not naming the suspect, said an FBI intelligence bulletin said the letters sent to Sen. Roger Wicker, R-Miss., and President Obama were postmarked Memphis, Tenn."
Our Original Post Continues:
A letter addressed to President Obama containing a substance that preliminary tests indicate was the deadly poison ricin has been intercepted at a remote mail facility, the FBI confirmed Wednesday.
Just a few miles away from the White House, suspicious packages delivered to two Senate office buildings brought bomb experts to the scene and forced police to tell staffers to remain in their offices. The packages were removed and after about an hour police gave the "all-clear."
The incidents follow Monday's bombings at the Boston Marathon and the discovery of another possibly poisoned letter — addressed to Republican Sen. Roger Wicker of Miss. — on Tuesday.
The FBI says there is no indication that the letters or packages are connected to the Boston bombings.
We're following the news. Scroll down to see our updates. (See this note about how we cover events such as this.)
12:50 p.m. ET Where Things Stand.
-- White House. A letter addressed to President Obama was intercepted at a remote mail handling facility on Tuesday and the FBI says preliminary tests indicated it may contain the poison ricin.
-- Senate. Meanwhile, police were investigating suspicious packages delivered to the Russell and Hart Senate office buildings on Capitol Hill. Those buildings were not been evacuated, but staffers were asked to avoid some areas. Capitol Police tell NPR's Tamara Keith that they were talking to an individual. By about 12:50 p.m. ET, authorities had removed the packages and issued an "all-clear."
-- Boston. Wednesday's incidents follow, of course, the bombing Monday at the Boston Marathon and word Tuesday that another letter, addressed to Republican Sen. Roger Wicker of Mississippi, preliminarily tested positive for ricin. According to the FBI, "the investigation into these letters remains ongoing, and more letters may still be received. There is no indication of a connection to the attack in Boston."
Update at 12:50 p.m. ET. All Clear In The Senate Buildings:
Police have removed the suspicious packages from the Russell and Hart Senate office buildings, CNN reports, and issued an "all-clear."
Update at 12:45 p.m. ET. What Is Ricin?
As we wrote Tuesday, the Centers for Disease Control and Prevention says "ricin is a poison found naturally in castor beans. ... If castor beans are chewed and swallowed, the released ricin can cause injury. Ricin can be made from the waste material left over from processing castor beans."
The Washington Post reported in 2004 that ricin is "twice as deadly as cobra venom."
Update at 12:30 p.m. ET: "A letter addressed to the president containing a suspicious substance" was received Tuesday at "the remote White House mail screening facility," the Secret Service confirmed Wednesday morning in an email to NPR's Ari Shapiro.
In early afternoon, the FBI issued a statement saying that the substance "preliminarily tested positive for ricin." And it added that "filters at a second government mail screening facility preliminarily tested positive for ricin this morning. Mail from that facility is being tested."
This follows the news from Tuesday that an envelope sent to the office of Sen. Roger Wicker, R-Miss., tested positive for the poison ricin in initial tests.
Update at 12:15 p.m. ET. Staff Told To Avoid Area.
This announcement was just made over the public address system in the Senate office buildings, NPR's Tamara Keith tells us:
"The U.S. Capitol police are continuing to investigate the suspicious packages in the first and third floors of the Hart senate office building. All staff and other personnel are to avoid this area until further notice."
Bear in mind: These type of situations do occur occasionally on Capitol Hill. Today's events are getting more attention, obviously, because they closely follow Monday's bombing at the Boston Marathon and the suspicious letter sent to Wicker.
Update at 12:07 p.m. ET. Package To Sen. Shelby:
A spokesman for Sen. Richard Shelby, R-Ala., says Capitol Police are investigating a "suspicious package" that arrived at the senator's office this morning, CNN says.
Update at noon ET. On The Police Activity On Capitol Hill:
Politico now writes that "the Secret Service Wednesday said it was investigating a suspicious substance that was found on a letter sent to President Barack Obama, as Capitol Police responded to reports of suspicious packages in the Hart and Russell Senate office buildings. A bomb squad is on the scene on Capitol Hill, but the buildings have not been evacuated. Certain areas of the buildings have been closed."
Update at 11:55 a.m. ET. Bomb Squad At Senate Office Buildings?
Politico tweets that "bomb squad [is] on the scene at Hart and Russell Senate office buildings" in Washington, D.C.
The Associated Press says "U.S. Capitol police are investigating the discovery of at least two suspicious envelopes in Senate office buildings across the street from the Capitol."
It was not clear whether there was a connection between the letter and the two envelopes.
Update at 11:45 a.m. ET. Similar To Letter Sent To Wicker, Source Tells AP:
"A law enforcement official said the letter [sent to Obama] is very similar to one recently mailed to Mississippi Republican Sen. Roger Wicker," The Associated Press reports. "The official requested anonymity because the official was not authorized to discuss the ongoing investigation."
Note: We will focus on reports from NPR, other news outlets with expertise, and statements from authorities who are in a position to know what's going on. And if some of that information turns out to be wrong, we'll update.