Paul Kevin Curtis was arrested at his home in Corinth, Miss., at 5:15 p.m. CDT Wednesday following an investigation by the FBI, the U.S. Capitol Police, the U.S. Postal Service and the U.S. Secret Service.
One letter was addressed to President Barack Obama and another was addressed to Mississippi Sen. Roger Wicker. Both were intercepted Tuesday at remote mail screening facilities.
Law enforcement officials say field tests are preliminary and are often unreliable. A complete lab test is being performed to positively determine what the substance is. That should be completed within the next 24 to 48 hours.
The FBI said the two letters were related and were both postmarked out of Memphis, Tenn., dated April 8.
In an intelligence bulletin obtained by The Associated Press, the FBI says the letters both say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both letters are signed, "I am KC and I approve this message."
Ricin is a potentially fatal poison.
There were reports of more suspicious mail. U.S. Capitol police investigated at least three suspicious packages sent to Senate office buildings. 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.
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 Thursday.
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.
The Associated Press contributed to this report.