CORINTH, Miss. -- A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obama and a senator. The letters 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 deadliest when inhaled.

An FBI intelligence bulletin said the two letters were postmarked Memphis, Tenn.

The 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."

In Corinth, a city of about 14,000 in northeastern Mississippi, police cordoned off part of a subdivision where Curtis may live. Police on the scene would not confirm why they had blocked a few roads and set up a crime scene in the area of brick, single-family homes. At least five police cars were on the scene.

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. Roger Wicker, R-Miss., arrives for a Senate Armed Services Committee hearing after a letter addressed to a him and poisoned with ricin or a similarly toxic substance was intercepted earlier this week at a mail facility outside Washington, on Capitol Hill in Washington, Wednesday, April 17, 2013. (AP Photo/J. Scott Applewhite)

All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday.

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.

All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday.

Senate Sergeant-at-Arms Terrance Gainer said that an individual who was responsible for the suspicious packages in the Hart and Russell Senate office buildings on Tuesday was detained and released on Wednesday. The packages were not hazardous.

Gainer said the man was "not particularly harmful, although terribly disruptive."

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.

Obama's press secretary, Jay Carney, said mail sent to the White House is screened at a remote site for the safety of the recipients and the general public. He declined to comment on the significance of the preliminary ricin result, referring questions to the FBI.

Capitol Police swiftly ramped up security, and lawmakers and staff were cautioned away from some parts of the Hill complex. After hours of jangled nerves, officials signaled it was safe to move throughout the area and people settled back to normal, if watchful, activity.

At a House hearing, Postmaster General Patrick Donahoe noted there had been ricin alerts since the notorious 2001 anthrax mailings and procedures are in place to protect postal employees and help track down culprits.

"Over the course of years we've had some situations where there have been ricin scares," Donahoe said. "Until this date, there's never been any actually proved that have gone through the system."

After Curtis' arrest, Wicker in a written statement thanked the FBI and U.S. Capitol Police for "their professionalism and decisive action in keeping our family and staff safe from harm."

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Associated Press writers Connie Cass, David Espo, Donna Cassata, Henry Jackson, Pauline Jelinek, Richard Lardner, Alan Fram, Ken Thomas, Jim Abrams, Andy Taylor, Seth Borenstein and Eric Tucker contributed to this report.