A Mississippi martial arts instructor was charged with possession of the biological agent ricin and with attempting to use it as a weapon, the US Department of Justice announced.
James Everett Dutschke, 41, was arrested by US marshals at his Tupelo home early on Saturday morning without incident, the city's police chief, Tony Carleton, said.
Agents from the FBI and the US Capitol Police, as well as members of an anti-terrorist response team from the Mississippi National Guard, some wearing hazardous material suits, had searched Dutschke's home on Tuesday and Wednesday, as well as the premises of a former martial arts studio Dutschke ran in the city.
Lori Basham, Dutschke's lawyer, did not return calls seeking comment on Saturday but told the Reuters news agency earlier in the week that her client denied having anything to do with the ricin letters.
She said Dutschke had been cooperating with federal officials during the searches this week.
US prosecutors dropped charges on Tuesday against another Mississippi man, Elvis impersonator Kevin Curtis, who was released from jail after a search of his home in nearby Corinth revealed no incriminating evidence.
Prosecutors said at the time that the investigation had "revealed new information" but provided no details.
Letters addressed to Senator Roger Wicker, a Mississippi Republican, and Obama were retrieved last week at off-site mail facilities before reaching their intended victims. A state judge also received a ricin-laced letter.
Ricin, which is made from castor beans, can be deadly to humans and is considered a potential weapon, particularly if refined into aerosol form.
Dutschke's name first surfaced in a federal court hearing on Monday for Curtis where his lawyer suggested her client had been framed by someone.
She mentioned a running feud between Dutschke and Curtis, albeit over a number of seemingly petty issues.
Suspicion had originally fallen on Curtis because of wording contained in all three ricin letters that appeared to incriminate him.
"Maybe I have your attention now/Even if that means someone must die," the letters read in part, according to the affidavit. The letters ended: "I am KC and I approve this message."
The initials "KC" led law enforcement officials to ask Wicker's staff if they were aware of any constituents with those initials, and the focus of the investigation then turned to Curtis, according to an affidavit from the FBI and the Secret Service filed in court.