UPDATE: The FBI announced Wednesday evening that it has arrested a man it suspects of sending the potentially ricin-laced letters to President Obama and GOP Sen. Roger Wicker (via the AP):
A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obama and a senator that tested positive for the poisonous ricin and set the nation's capital on edge a day after the Boston Marathon bombings.
FBI Special Agent in Charge Daniel McMullen said Paul Kevin Curtis, 45, was arrested at 5:15 p.m. at his apartment in Corinth, near the Tennessee state line about 100 miles east of Memphis. It wasn't immediately known where he was being held.
According to previous reports, both letters were signed: "I am KC and I approve this message."
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Original Post 11:45 a.m.: Washington's ongoing ricin scare just went up a notch. Politico:
A letter addressed to President Obama containing what the Secret Service calls "a suspicious substance" was intercepted Tuesday at a mail screening facility, the agency said Wednesday.
Spokesman Edwin Donovan said the letter was received at the Secret Service's White House mail screening facility, which examines all mail for the complex and is not located nearby. The facility, he said, "facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery."
There's been no official confirmation that the substance in question is ricin, although sources tell ABC News that field-tests (which aren't always the most reliable) suggest that it was. The news does come one day after we learned that someone sent a letter with a substance that tested positive for ricin or a similar poison to the office of GOP Sen. Roger Wicker of Mississippi. That letter, however, was also intercepted at an off-site screening facility and never made it to Capitol Hill.
Ricin is a poison made from castor beans that can kill you if ingested. According to the CDC, it comes in several forms, including powder and pellets, and can be dissolved in water. It's perhaps most well-known as the poison used in the assassination of the Bulgarian dissident Georgi Markov. The news of the ricin-laced letter is sure to spark memories of 2001, when several lawmakers received letters containing what is believed to have been anthrax.
This post has been updated with additional information as it became available.