A man has been arrested after letters suspected of containing the poison ricin were sent to President Barack Obama and a US Senator.
The alleged sender is believed to be Paul Kevin Curtis, a 45-year-old from Mississippi, according to the FBI.
Initial tests showed the letters, sent to Obama and Mississippi senator Roger Wicker, contained traces of ricin, which is known to cause death within three to five days of exposure.
A third letter is also alleged to have been sent by Curtis to a Mississippi justice official.
The FBI has said there was no connection to the sending of these letters and the Boston marathon explosion which left three people dead.
The letters read: "To see a wrong and not expose it, is to become a silent partner to its continuance" and signed off "I am KC and I approve this message".
Both letters were postmarked Memphis, Tennessee, and dated 8 April.
The letter to Obama was intercepted by the mail room at Congress in Washington.
The FBI said: "Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests are currently being conducted and generally take 24 to 48 hours."
Preliminary field tests can often show false positives for ricin.
Senator Carl Levin said a member of staff at his office in Michigan would spend the night in hospital as a precaution after discovering a suspicious letter.
Levin confirmed the person concerned had no symptoms and expected to learn preliminary results of tests on the letter today (18 April).
Ingestion of ricin can cause nausea, vomiting, diarrhoea, gastric haemorrhaging and shock. With a sufficient dose, death can occur within three to five days.
Other symptoms of exposure to ricin can include internal bleeding and major organ failure.