Everett Dutschke, 41, was taken into custody by U.S. marshals at his Tupelo home early Saturday morning without incident, the city's police chief, Tony Carleton, told Reuters.
It was not immediately known if Dutschke has been charged in the ricin investigation.
Dutschke faces other charges related to an April 1 indictment for fondling three different children between ages 7 and 16, from 2007 to 2013, according to court records. He was released on $25,000 bond in that case.
Dutschke's attorney, Lori Basham, did not return calls seeking comment but told Reuters earlier in the week that her client denied having anything to do with the ricin letters.
Agents from the FBI and the U.S. 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.
Dutschke was cooperating with federal officials during the searches this week, the attorney said.
The agents had Dutschke's home under surveillance on Friday afternoon and evening and moved to arrest him about 1 a.m. CDT (0600 GMT).
U.S. 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.
The case has brought extra scrutiny on the FBI almost 12 years after a 2001 letter-borne anthrax attack in the wake of the September 11 attacks in New York and Washington. It took investigators seven years to solve the anthrax case.
Letters addressed to Senator Roger Wicker, a Mississippi Republican, and Democratic President Barack Obama were retrieved last week at off-site mail facilities before reaching their intended victims. A state judge also received a ricin-laced letter.
The discovery added another layer of anxiety as authorities dealt with bombings at the Boston Marathon.
Ricin, which is made from castor beans, can be deadly to humans and is considered a potential terror weapon, particularly if refined into an aerosol form.
Dutschke's name first surfaced in a federal court hearing on Monday for Curtis where his attorney 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.