Francis O'Neill, a former psychiatric nurse, was found dead in his home in Omagh early last Saturday. He lived alone.
A 15-year-old boy was arrested in Belfast last night.
A 16-year-old juvenile and an 18-year-old man who had originally been held for burglary of Mr O'Neill's house were re-arrested yesterday.
Police say they will increase patrols in the Brook Valley area of Omagh where Mr O'Neill lived. His house had been ransacked.
