Elytte Barbour told investigators the newlyweds killed LaFerrara because they wanted "to murder someone together," police said.
Defense lawyers are seeking psychiatric evaluations for the Barbours, who have pleaded not guilty.
Miranda Barbour's mother, Elizabeth Dean, said in an off-camera interview Tuesday with WRAL-TV in Raleigh, N.C., that she couldn't imagine her committing the previous killings. Dean, of Cary, N.C., said she and her daughter had moved from Alaska to North Carolina in 2012 to get a fresh start.
Sunbury police Chief Steve Mazzeo has said investigators are aware of Miranda Barbour's claims about killings in other states and were contacting police in those jurisdictions.
Neither Pennsylvania state police nor the Pennsylvania attorney general's office is involved in the investigation, spokesmen said Tuesday.
North Carolina's State Bureau of Investigation "has been in contact with Pennsylvania authorities about this case" and remains in contact "to determine if there is any credible information related to any unsolved homicide in North Carolina," spokeswoman Noelle Talley said.
In Alaska, state troopers said they were "not aware of any information, beyond Barbour's comments quoted in the press, or evidence that would implicate Barbour with a homicide committed in Alaska."
Associated Press writers Rachel D'Oro in Anchorage, Alaska, and Martha Waggoner in Raleigh, N.C., contributed to this report.