U.S. attorney Felicia Adams and Daniel McMullen, the FBI agent in charge in Mississippi, made the announcement in news release Saturday following the arrest of 41-year-old James Everett Dutschke.
FBI spokeswoman Deborah Madden said Dutschke (pronounced DUHS'-kee) was arrested about 12:50 a.m. Saturday at his house in Tupelo.
The letters, which tests showed were tainted with ricin, were sent April 8 to President Barack Obama, Sen. Roger Wicker of Mississippi and 80-year-old Mississippi judge, Sadie Holland.
Dutschke is expected to appear Monday in U.S. District Court in Oxford.
He faces up to life in prison, if convicted.
The news release said 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."
Dutschke's house, business and vehicles were searched earlier in the week and he had been under surveillance.
Dutschke's attorney, Lori Nail Basham, said Saturday in a text message that "the authorities have confirmed Mr. Dutschke's arrest. We have no comment at this time."
Basham said earlier this week that Dutschke was "cooperating fully" with investigators. Dutschke has insisted he had nothing to do with the letters.
Ryan Taylor, a spokesman for Wicker, said Saturday that "because the investigation is still ongoing, we're not able to comment."
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 told The Associated Press, "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," Dutschke said.
Charges were dropped against, Paul Kevin Curtis, 45, the Elvis impersonator, after authorities said they had discovered new information. Curtis' lawyers say he was framed.
Curtis' attorney, Christi McCoy, said Saturday: "We are relieved but also saddened. This crime is nothing short of diabolical. I have seen a lot of meanness in the past two decades, but this stops me in my tracks."
Some of the language in the letters was similar to posts on Curtis' Facebook page and they were signed, "I am KC and I approve this message." Curtis' signoff online was often similar.
Dutschke and Curtis were acquainted. Curtis said they had talked about possibly publishing a book on an alleged conspiracy to sell body parts on a black market. But he said they later had a feud.
Judge Holland is a common link between the two men, and both know Wicker.
Holland was the presiding judge in a 2004 case in which Curtis was accused of assaulting a Tupelo attorney a year earlier. Holland sentenced him to six months in the county jail. He served only part of the sentence, according to his brother.
Holland's family has had political skirmishes with Dutschke. Her son, Steve Holland, a Democratic state representative, said he thinks his mother's only other encounter with Dutschke was at a rally in the town of Verona in 2007, when Dutschke ran as a Republican against Steve Holland
Holland said his mother confronted Dutschke after he made a derogatory speech about the Holland family. She demanded that he apologize, which Holland says he did.
On Saturday, Steve Holland said he can't say for certain that Dutschke is the person who sent the letter to his mother but added, "I feel confident the FBI knows what they are doing."
"We're ready for this long nightmare to be over," Holland told The Associated Press.
He said he's not sure why someone would target his mother. Holland said he believes Dutschke would have more reason to target him than his mother.
"Maybe he thinks the best way to get to me is to get to the love of my life, which is my mother," Holland said Saturday.
Associated Press writer Jack Elliott Jr. in Jackson contributed to this report.