OXFORD, Miss. — A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obawma 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 the man was arrested Wednesday. His name wasn’t immediately released publicly.

Authorities still waited for definitive tests on the letters to Obama and Sen. Roger Wicker, R-Miss. An FBI intelligence bulletin obtained by The Associated Press said those 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.”

The letters were intercepted before reaching the White House or Senate. The FBI said Wednesday that more testing was under way. Preliminary field tests can often show false positives for ricin.

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, a fire department spokesman said.

All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday. But a man was still being questioned after being stopped in connection with the packages, she said.

All the activity came as tensions were high in Washington and across the country following Monday’s bombings at the Boston Marathon that killed three people and injured more than 170.

The FBI said there was no indication of a connection between the letters and the bombing. The letters to Obama and Wicker were postmarked April 8, before the marathon.