A 79-year-old man was left in a critical condition after going to investigate a sighting of two men in the grounds of his Aberdeenshire home.
Sandy Ingram, who lives in a remote location in Newmachar, was later found lying in his garden with head injuries.
Grampian Police said the men would have needed a vehicle to get to the man's property.
They have appealed for anyone who was in the area on Saturday at about 2300 BST to get in contact.
Det Insp Richie Guild said the force was conducting door-to-door inquiries in the area.
He said: "We're continuing our investigations into the circumstances surrounding how Sandy came by his injuries.
"It's important to stress that we have as yet been unable to speak to Sandy to establish exactly what happened.
"Until we get that opportunity, we are keeping an open mind."
He explained: "We are following up several lines of enquiry, including suspicious vehicles in the area around the time of the incident.
"If anyone in or around Newmachar has seen vehicles in suspicious circumstances, particularly in rural areas, they should contact police as soon as possible."
The pensioner was taken to Aberdeen Royal Infirmary, where his condition was described as "critical but stable".
One of the two men is described as wearing a light-coloured baseball cap, possibly green, with a darker peak.
The other was said to be wearing dark clothing and had long, almost shoulder length, dark hair.
Det Insp Guild said: "We are very keen to speak to these two men, and if anyone has seen people matching the descriptions in the Newmachar area recently we'd ask them to pick up the phone and call us."