Rodney Sanders, 46, of Kosciusko, Mississippi, is charged with capital murder over the deaths of Sister Margaret Held and Sister Paula Merrill, both 68.
Willie March, the sheriff of Holmes County said he had been briefed by police from the town of Durant where the killings occurred and Mississippi Bureau of Investigation officials who took part in Sanders’ interrogation.
Sanders confessed in the interrogation to the killings but gave no reason for the crimes, said March.
The sheriff said police work and tips from the community led police to Sanders.
Sanders was convicted last year of a driving under the influence offence, said Grace Fisher, a spokeswoman for the Mississippi Department of Corrections. He was later released from prison and is currently on probation.
He was also convicted of armed robbery in Holmes County, sentenced in 1986 and served six years, Ms Fisher said.
People who knew the nuns, known for their generosity and commitment to improving health care for the poor, have been grappling with why anyone would want to kill them.
Dr Elias Abboud, who oversees the clinic in Lexington where the nuns worked, said Sanders was not a patient there.
The women’s bodies were discovered on Thursday after they failed to arrive for work in Lexington, about 10 miles from where they lived.
The sheriff said they had been stabbed.
“Sanders was developed as a person of interest early on in the investigation,” police lieutenant colonel Jimmy Jordan said.
Authorities said Sanders was being held in a detention centre pending a court appearance.
Mississippi Department of Public Safety spokesman Warren Strain said investigators believed Sanders acted alone.
Sister Paula’s nephew, David Merrill, speaking from Stoneham, Massachusetts, said the family was “thankful” Sanders was off the streets.
Merrill said he agreed with the idea of forgiveness and that was something his aunt would want for whoever killed her, but he wasn’t sure if he was capable of completely forgiving.
He said he would not support the death penalty if Sanders were to be convicted. The capital murder charge leaves open the possibility Sanders would face the death penalty but that determination would be made by prosecutors later.
Meanwhile, in the poverty-stricken Mississippi county where the nuns were slain, many people were still mourning their loss.
Jonell Payton, a Durant alderwoman, who lives a few doors from the nuns’ house, said they were “the most precious two people” and known for helping provide medicine for those who could not afford it.
Both women worked at the clinic, where they gave flu shots, dispensed insulin and provided other medical care for children and adults.