OXFORD, Miss. — A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obama and a senator that tested positive for poisonous ricin and set the nation’s capital on edge a day after the Boston Marathon bombings.

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, said FBI Special Agent in Charge Daniel McMullen. It wasn’t immediately known where he was being held.

Authorities still waited for definitive tests on the letters to Obama and Sen. Roger Wicker, R-Miss. Preliminary field tests can often show false positives for ricin. The letters were intercepted before reaching the White House or Senate. Ricin is derived from the castor plant that makes castor oil. There is no antidote and it is at its deadliest when inhaled.

An FBI intelligence bulletin obtained by The Associated Press said the two letters were postmarked Memphis, Tenn.

Both letters said: “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.”

As authorities scurried to investigate three questionable packages discovered in Senate office buildings, reports of suspicious items also came in from at least three senators’ offices in their home states.

Sen. Carl Levin said a staff member at his Saginaw, Mich., office would spend the night in a hospital as a precaution after discovering a suspicious letter. The staff member had no symptoms, Levin said in a statement. He expected to learn preliminary results of tests on the letter by today.

Sen. Jeff Flake, R-Ariz., said suspicious letters at his Phoenix office had been cleared with nothing dangerous found. A package at Sen. John Cornyn’s Dallas-area office also was declared harmless.