A man is still being questioned today about the murder of retired librarian in Co Tyrone.
Mrs Attracta Harron (65) went missing on her way home from mass in Lifford, Co Donegal, more than three months ago.
Police yesterday searched the home of the 21-year-old unemployed man from the Sion Mills area near Strabane, Co Tyrone.
Forensic tests have also been carried out on the remains of a burned-out car found soon after Mrs Harron's disappearance. Even though her body has not been recovered, police are treating the case as murder.
Mrs Harron's husband, Michael (65), a retired schoolteacher, said today he did not want to make any comment on the latest stage of the police investigation.
A spokeswoman for the Police Service of Northern Ireland said: "Police are keeping an open mind as to the motive but it's not believed to be sectarian."
Detectives began a murder inquiry after failing to find Mrs Harron's body during extensive searches of three rivers in the Strabane and Lifford areas. They are trying to find the drivers of two cars that were spotted around the time she is believed to have gone missing.
One of them was seen speeding away from the Sion Mills area towards the Donegal border with a woman in the front passenger seat who was reportedly bleeding from a head wound.
The other had been parked between Lifford and Strabane close to where a woman who fitted Mrs Harron's description was seen standing beside it.