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Update- CBS news reports an arrest has been made in the letters sent to President Obama and Mississippi Senator Roger Wicker.

A man from Tupelo, MS has been taken into custody for sending letters containing Ricin.

(CNN) The FBI confirms a letter containing what appears to be the poison Ricin has been sent to President Obama.

The FBI confirms the letter was postmarked from Memphis on April 8.

A similar letter, also from Memphis, was sent to Mississippi Senator Roger Wicker and also contained the poison Ricin.

“To see a wrong and not expose it, is to become a silent partner to its continuance.”

The letters were signed “I am KC and I approve this message,” the source said.

Discovered Tuesday, the letters were addressed to President Barack Obama and to Sen. Roger Wicker, R-Mississippi. Because initial tests can be “inconsistent,” the envelopes have been sent off for additional tests, an FBI statement said. Those results were expected later Wednesday, an FBI official told CNN.

Preliminary tests on filters at a government mail-screening facility also indicated the presence of ricin Wednesday morning, and mail from that site also was being tested, the FBI said.

Reports of suspicious packages and envelopes also came into two Senate office buildings late Wednesday morning. Capitol Police evacuated the first floor of the Hart Senate Office Building for more than an hour and questioned a man in the area who had a backpack containing sealed envelopes, but the man was not taken into custody.

“It just reminds you that with public service comes the real possibility that you could be a target,” said Sen. Pat Roberts, R-Kansas. “But on the other side of it, we have an excellent police force, and I think they’ll get to the bottom of it.”

Meanwhile, Sen. Carl Levin, D-Michigan, says one of his home-state offices received a “suspicious-looking” letter and alerted authorities. “We do not know yet if the mail presented a threat,” said Levin, the chairman of the Senate Armed Services Committee.

A staffer for Arizona Sen. Jeff Flake flagged “suspicious letters” at the freshman Republican’s Phoenix office, Flake spokeswoman Genevieve Rozansky said in a statement, but “no dangerous material was detected in the letters.”

Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained some type of powder. The person who initially found the envelope is being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were examined as well.

In a statement issued Wednesday, the FBI said it has no indication of a connection between the tainted letters and Monday’s bombings at the finish line of the Boston Marathon. But the discoveries further heightened security concerns at a time when Congress is considering politically volatile legislation to toughen gun laws and reform the immigration system.

White House spokesman Jay Carney said the president had been briefed on the letters.

“Obviously, he understands and we all understand that there are procedures in place, as the FBI has said. There are — there is is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now,” Carney said.

The line in the letters about exposing “a wrong” comes from John Raymond Baker, a longtime Texas chiropractor, his wife said. It’s been widely quoted online, but Tammy Baker sounded surprised that it was used in the letters under scrutiny in Washington.

She said the phrase refers to her husband’s general philosophy of care. She said the office phone started ringing frequently Wednesday afternoon, and it was “kind of freaking out our other employee.”

Mail for members of Congress and the White House has been handled at offsite postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota.

The letter sent to Wicker had a Memphis, Tennessee, postmark and no return address, Senate Sergeant-at-Arms Terrance Gainer wrote in an e-mail to senators and aides Tuesday. Wicker has been assigned a protective detail, according to a law enforcement source.

A laboratory in Maryland confirmed the presence of ricin on the letter addressed to Wicker after initial field tests also indicated the poison was present, according to Gainer. However, the FBI said additional testing was needed because field and preliminary tests produce inconsistent results.