UPDATE 6:36 p.m.: A Mississippi man was arrested Wednesday, accused of sending letters to President Barack Obama and a senator that tested positive for the poisonous ricinand set the nation's capital on edge a day after the Boston Marathon bombings.

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

UPDATE 5:09 p.m.: A man in Mississippi has been arrested and accused of sending letters with suspected ricin poison to President Barack Obama and other leaders.

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., which had raised concern Wednesday at a time when many people were jittery after the Boston bombings.

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

UPDATE 10:25 a.m.: Letters sent to President Barack Obama and a Mississippi senator that tested positive for poisonous ricin are related and both are postmarked Memphis, Tenn., the FBI said Wednesday. A senator said police have a suspect in mind. Several other reports of suspicious mail to government officials were being checked.

Authorities are also investigating two suspicious letters that were sent to the Phoenix office of U.S. Sen. Jeff Flake of Arizona.

Emergency crews in hazardous materials gear were seen outside the building Wednesday morning.

U.S. Sen. John McCain has an office directly across the street, and a spokesman for him said authorities have told staff not to open any packages as authorities investigate.

In an intelligence bulletin obtained by The Associated Press, the FBI says that letters to Obama and Sen. Roger Wicker, R-Miss., 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."

The activity came as tensions were high in Washington and across the country following the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170. The FBI said there is no indication of a connection between the letters and the bombing. The letters to Obama and Wicker were postmarked April 8, before the marathon.

Sen. Claire McCaskill, D-Mo., said that police suspect a person who "writes a lot of letters to members." She made the comment Tuesday as she emerged from a briefing by law enforcement on the Boston bombing. Authorities declined to comment on a suspect.

In addition to the letters, U.S. Capitol police were investigating the discovery of at least three suspicious packages in Senate office buildings.

Senate Sergeant at Arms Terence Gainer said in an email that packages were dropped off at the offices of two senators, and Sen. Richard Shelby, R-Ala., said in a statement his office had received one of them.

A third package was found in an atrium on the first floor of one of the two buildings. A person who delivered at least two of the packages was being questioned, Gainer said, as Capitol police swiftly ramped up security.

Both the letters to Wicker, R-Miss., and to Obama were intercepted at off-site mail facilities.

Separately, Sen. Carl Levin of Mich., issued a statement saying an aide in his Saginaw, Mich. Office had received a suspicious-looking letter. "The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating," the Michigan Democrat said in a statement.

The discoveries spread concern in the sprawling Capitol complex, and authorities swiftly stepped up their security presence.

In one case, police sealed off a hearing room where Defense Secretary Chuck Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of staff, were testifying.

In another, officers advised Sen. Joe Manchin and aides not to board an elevator because suspicious packages had been found on several floors of the Hart Office Building.

"They just told me there's something suspicious and they're looking into it," Manchin said.

The FBI said the letters to Obama and Wicker were undergoing further testing. Preliminary testing can be unreliable, showing false positives for ricin.

Around the Capitol there was an increased police presence Wednesday. Outside, many public garbage cans were emptied and turned on their side. Yet public tours of the building continued as usual.

UPDATE 9:22 a.m.: The FBI says preliminary tests on a letter sent to President Barack Obama indicate the presence of poisonous ricin.

The letter is undergoing further testing because preliminary field tests can be unreliable, creating false positives.

The letter was intercepted at a facility away from the White House. It comes the day after officials said a letter sent to Sen. Roger Wicker tested positive for poisonous ricin. That letter to Wicker, a Republican, was intercepted at a Senate mail facility just outside Washington.

The FBI says there is no indication of a connection to the bombing at Monday's Boston Marathon.

PREVIOUSLY: The U.S. Secret Service says it has intercepted a letter addressed to President Barack Obama that contained a "suspicious substance." And police are investigating at least two suspicious packages in Senate office buildings.

Secret Service spokesman Ed Donovan says the letter was intercepted at a facility away from the White House. He says the letter was received Tuesday.

The letter comes a day after lawmakers said a letter was mailed to Sen. Roger Wicker that tested positive for poisonous ricin. Another senator said police have a suspect in mind.

Tensions have been high in Washington and across the country since the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170.

Also in Washington D.C., U.S. Capitol police are investigating the discovery of at least two suspicious packages in Senate office buildings and "the individual who delivered them is being questioned," Sergeant at Arms Terence Gainer said on Wednesday.

Gainer said in an email the packages were delivered to at least two Senate offices.

The development comes on the day after a letter mailed to Sen. Roger Wicker of Mississippi was found in two preliminary tests to contain ricin, a potentially fatal poison. The letter was intercepted at an off-site facility where all mail to the Senate is screened.

It was not clear whether there was a connection between the letter and the envelopes.

Other officials said one of the packages had been discovered in the Russell Building, and the second in the Hart office building.

KPCC will update this story as we get more information.