The ricin-laced letters sent to President Obama and Mississippi Sen. Roger Wicker were both postmarked April 8 and sent from Memphis, Tenn., signed "I am KC and I approve this message." A third letter went to Michigan Sen. Carl Levin. NBC's Andrea Mitchell reports.

Federal agents on Wednesday arrested a suspect in the mailing of letters to President Barack Obama and a U.S. senator that initially tested positive for the poison ricin.

The suspect was identified as Paul Kevin Curtis of Tupelo, Miss., federal officials told NBC News. They said he may appear in court as early as Wednesday night.

Both letters carried an identical closing statement, according to an FBI bulletin obtained by NBC News on Wednesday.

According to the FBI bulletin, both letters, postmarked April 8, 2013 out of Memphis, Tenn., included an identical phrase, "to see a wrong and not expose it, is to become a silent partner to its continuance."

In addition, both letters are signed: "I am KC and I approve this message."

The letter to Obama was intercepted at an off-site White House mail facility and was being tested further, the FBI said. A federal law enforcement official said that the letter was “very similar” to one addressed to Sen. Roger Wicker, R-Miss. Another letter was addressed to a Mississippi justice official.

Lee County Sheriff Jim Johnson confirms a suspect has been arrested in relation to the ricin-laced letter sent to a Mississippi judge. Johnson adds that there are "great consistencies" between this letter and those sent to President Obama and Senator Wicker.

Curtis was arrested at his home in Corinth, Miss., at about 5:15 CT, after an investigation by federal, state and local agencies.

In a news conference Wednesday night, Lee County, Miss., Sheriff Jim Johnson said a Mississippi judge on April 10 received and opened a typewritten letter -- postmarked from Memphis, but without a return address -- that included “wording that was of interest,” as well as some "suspicious content."

Johnson said there were “great consistencies” between the letter received by the Mississippi judge and the letters directed to Wicker and President Obama. Tests are being conducted on the letter sent to the Mississippi judge to determine whether it was tainted with ricin, Johnson added.

The FBI is assisting the sheriff’s office in the investigation to determine whether the letters were sent by the same person.

Johnson would not identify the suspect in custody, adding that local authorities are waiting for results from the federal laboratory before filing any state charges.

Two federal officials said late Wednesday that an initial laboratory test on the material in the letters was inconclusive. The test shows some level of ricin, they said, but the potency is uncertain. They cannot tell whether the material is actually harmful or not, so more tests have been ordered.

Wicker released a statement Wednesday, thanking authorities for their help.

"Gayle and I want to thank the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm," the statement read.

"My offices in Mississippi and Washington remain open for business to all Mississippians. We particularly want to thank the people of Mississippi for their thoughts and prayers during this time."

The sender of the letters, one official said, "may have stumbled onto something," but it's unknown if he actually made the full-blown ricin toxin.

Ricin is made from castor beans and can kill within 36 hours. There is no antidote. Some threatening letters simply contain ground castor beans, resulting in a positive field test for ricin without the concentrated poison. Results from full laboratory tests are expected in the next 24 to 48 hours.

Filters at a second government mail screening facility also tested positive for ricin in a preliminary screening Wednesday.

An FBI official told NBC News that the agency did not initially believe the letters were related to the attack on the Boston Marathon on Monday.

Authorities also for a time cleared the atrium of a Senate office building Wednesday, removing suspicious envelopes and a package, before reopening the offices. Capitol police were also investigating a suspicious package at the office of Sen. Richard Shelby, R-Ala. Shelby’s staff had not been evacuated.

The Wicker letter had no return address. The FBI confirmed the preliminary positive test on it Tuesday. That letter was intercepted at a postal facility in Maryland that screens mail sent to Congress, and never reached Wicker’s office.

Other senators were made aware of the Wicker letter during a briefing Tuesday evening on the bombing in Boston. Sen. Claire McCaskill, D-Mo., said that the person who sent Wicker the letter writes often to elected officials.

People can be exposed to ricin by touching a ricin-laced letter or by inhaling particles that enter the air when the envelope is opened. Touching ricin can cause a rash but is not usually fatal. Inhaling it can cause trouble breathing, fever and other symptoms, and can be fatal.

At a hearing Wednesday on the Postal Service’s finances, Postmaster General Patrick Donahoe said that while there have been ricin scares in the past, the recent discoveries were unprecedented.

“There's never been any actually proved that have gone through the system,” Donahoe said. “But we've got a process that we make sure that our employees know -- We can actually track the mail back through the system to double check from an employee health standpoint."

Field tests are conducted anytime suspicious powder is found in a mail facility, and the FBI cautioned that field tests and other preliminary tests can produce inconsistent results. When tests show the possibility of a biological agent, the material is sent to a laboratory for full analysis.

Robert Windrem, Kasie Hunt, Kelly O’Donnell, Richard Esposito, Jeff Black, Mike Viqueira and Dr. Kristina Krohn of NBC News contributed to this report.

Editor's note: An earlier version of this story misidentified the suspect, based on information from federal officials.

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