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Paul Kevin Curtis was arrested at his home in Corinth, Mississippi, and is "believed to be responsible for the mailings of the three letters sent through the US Postal Inspection Service which contained a granular substance that preliminarily tested positive for ricin," the Justice Department said in a statement.
The letters were addressed to a US senator, the White House and a Mississippi justice official, the statement said.
The ricin poison scare hit Washington after bombings at the Boston Marathon killed three people and injured 176 on Monday, but the FBI said there was no indication the incidents were connected.
The FBI said the envelope sent to Obama was received at a mail-screening facility outside the White House and was immediately quarantined. Preliminary tests showed it contained the deadly poison ricin, the FBI said
Washington was put on edge on Tuesday evening when news emerged that authorities had intercepted a letter sent to Republican Senator Roger Wicker of Mississippi that had initially tested positive for ricin.
Following the arrest, Wicker issued a statement thanking the FBI and Capitol Police "for their professionalism and decisive action in keeping our family and staff safe from harm."
Earlier on Wednesday, a flurry of reports of suspicious letters and packages rattled the US capital and caused the temporary evacuation of parts of two Senate buildings. Most of the reports quickly proved to be false alarms, and business was only temporarily disrupted on Capitol Hill.
The letters to Obama and Wicker had identical language, included the phrase, "To see a wrong and not expose it, is to become a silent partner to its continuance." They were signed, "I am KC and I approve this message," according to an FBI operations bulletin reviewed by Reuters.
The envelopes both bore postmarks from Memphis, Tennessee, and were dated April 8. Memphis Mayor A.C. Wharton noted in a statement, however, that it did not mean the letters originated in that city.
An aide to Wharton said many areas near Memphis were included in its postmark - including some in neighboring northern Mississippi, Wicker's state.
For Washingtonians, the situation was an unsettling reminder of events of nearly 12 years ago when letters containing anthrax spores were mailed to the Washington offices of two senators and to media outlets in New York and Florida, not long after the Sept. 11, 2001, attacks in New York and Washington.
The FBI said White House operations were not affected by the latest scare. It noted that filters at a second government mail-screening facility had preliminarily tested positive for ricin this morning" and mail from that facility was also being tested.
The tests were being conducted at Fort Detrick, in Frederick, Maryland, a government source said.
White House spokesman Jay Carney said Obama had been briefed on the situation.
Ricin is a lethal poison found naturally in castor beans, but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.
There was another ricin scare at the US Capitol in 2004, when tests showed positive on a letter in a Senate mail room that served the office of Bill Frist, a Tennessee Republican who was then Senate majority leader.
SERIES OF SUSPICIOUS ITEMS
Law enforcement authorities on Wednesday closed and then reopened parts of the Hart and Russell Senate buildings near the Capitol after tests on suspect items showed there was no threat.
"All test results were negative," US Capitol Police said over the public address system in Senate office buildings.
Police questioned a man with a backpack who had been delivering envelopes to Senate offices, a law enforcement official said. This delivery method broke the normal protocol, because no mail is supposed to be delivered without first being checked at an outside facility, Capitol officials said.
Police also investigated a suspicious package delivered to Alabama Republican Senator Richard Shelby's office in the Russell building on Wednesday, but determined there was no threat, said Jonathan Graffeo, a spokesman for Shelby.
West Virginia Democratic Senator Joe Manchin told reporters he was kept out of his own office in the Hart office building for an hour as officials investigated something deemed suspicious.
In Arizona, the bureau of Alcohol, Tobacco, Firearms and Explosives said two suspicious letters had been sent to Republican Senator Jeff Flake's Phoenix office. Two staffers and a police officer were taken to the hospital as a precautionary measure after reporting irritation when handling them.
Flake later issued a statement on Wednesday saying no dangerous materials were detected in the mailings. One of them originated in Tennessee, Flake told reporters outside the Senate.
In Ohio, Columbus police responded to a report of a suspicious letter received on Wednesday at Republican Senator Rob Portman's office, but determined it was not dangerous, Portman's office said.
Democratic Senator Carl Levin said one of his Michigan regional offices had also received a suspicious letter, but it was not opened. Authorities are investigating, and a staff member went to the hospital as a precautionary measure, he said.
It is unclear if there was a connection linking the series of suspicious items delivered to politicians.
The Senate's sergeant at arms, Terrance Gainer, sent a memo to all offices telling them only to accept mail from a uniformed Senate post office employee and, when in doubt, to call the police.
He said mail was being delivered that had already been cleared, but there would be no mail delivered on Thursday and Friday to allow more testing and investigation.