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