A Mississippi man has been charged with making and possessing ricin in the investigation into poison-laced letters sent to President Barack Obama and others.
US attorney Felicia C Adams and Daniel McMullen, the FBI agent in charge in Mississippi, made the announcement following the arrest of 41-year-old James Everett Dutschke.
FBI spokeswoman Deborah Madden said Dutschke was arrested at his house in Tupelo.
The letters, which tests showed were tainted with ricin, were sent on April 8 to Mr Obama, Senator Roger Wicker of Mississippi and 80-year-old Mississippi judge Sadie Holland.
Dutschke is expected to appear on Monday in US District Court in Oxford, Mississippi.
He faces life in prison if convicted. His lawyer said in a text that she had no comment.
Dutschke was charged with "knowingly developing, producing, stockpiling, transferring, acquiring, retaining and possessing a biological agent, toxin and delivery system, for use as a weapon, to wit - ricin."
His house, business and vehicles were searched earlier in the week and he had been under surveillance.
Dutschke's lawyer, Lori Nail Basham, said in a text message that "the authorities have confirmed Mr Dutschke's arrest. We have no comment at this time." She said earlier this week that Dutschke was "co-operating fully" with investigators. Dutschke has insisted he had nothing to do with the letters.
Charges in the case were initially filed against an Elvis impersonator but then dropped. Attention then turned to Dutschke, who has ties to the former suspect, the judge and the senator. Earlier in the week, as investigators searched his primary residence in Tupelo, Dutschke said: "I don't know how much more of this I can take. I'm a patriotic American. I don't have any grudges against anybody. ... I did not send the letters."