Suspect arrested for ricin-positive letters sent to Obama

The FBI said it arrested on Wednesday Paul Kevin Curtis of Corinth, Mississippi, in connection with ricin letters sent to U.S. officials, including President Barack Obama.

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.

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.

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