A Mississippi man is under arrest in connection with letters containing poison that were sent to President Barack Obama and a U.S. Senator.
Federal agents say 45-year-old Paul Kevin Curtis was arrested at his home in Corinth, Mississippi.
The envelopes were postmarked in nearby Memphis, Tennessee.
Preliminary tests indicate that the substance in the envelopes is ricin, which is deadly in very small quantities.
Mississippi Sen. Roger Wicker, a Republican, received a letter that FBI officials say is connected to the one sent to Obama.
The FBI says both letters had similar cryptic messages and were signed with the initials K.C.
Experts are conducting more definitive tests on the letters, which were intercepted at a Maryland mail screening depot.
The FBI said there's no indication the letters are connected with the attack in Boston.