Police say they are questioning a "person of interest" in connection with a massive search underway at a rural Airdrie, Alta., property that they have linked to the disappearance of a five-year-old Calgary boy and his grandparents.

RCMP and Calgary police have been conducting a massive search of a rural property northeast of Calgary since Friday night, in search of clues to the disappearance of Nathan O'Brien, and Kathy and Alvin Liknes, who were reported missing Monday.

RCMP tactical teams and the Calgary Police Service only confirm at 5:40 p.m. MT Saturday that combing the property is linked to the search for the three family members.

Police Nathan car

This Ford truck has been linked to the disappearance of a five-year-old Calgary boy and his grandparents. Police have been searching a massive property in Airdrie, Alta., in connection with the disappearance of Nathan O'Brien and Kathryn and Alvin Liknes. (Calgary Police Service)

At a news conference at 6:30 p.m., they said a man from that residence has been taken in for questioning and is co-operating "to a degree." 

They have also taken a vehicle from the property that is similar in description to the one captured on CCTV images released Friday.

The Amber Alert for Nathan remains in effect as neither he nor the Liknes's have been found, and police expected to remain at the property for the next 24 to 36 hours.

Earlier Parkhill ground search over

Earlier Saturday, a new ground search took place around the Liknes's Parkhill home, but that ended without explanation.

The search area included a 400-metre perimeter around the Liknes's home. About 50 search team members examined homes, vehicles and buildings, and interviewed anyone they could find.

Nathan, along with his grandparents, were discovered missing Monday at 10 a.m. when Nathan's mother arrived to pick him up from a sleepover at their house.

The couple had held an estate sale on Friday, Saturday and Sunday in preparation for a move to Edmonton and the O'Brien family had been over helping out until they left at 10 p.m. Sunday, leaving Nathan to spend the night.

On Friday, police said in a news conference there is no indication of forced entry to the home but blood was found inside the home and police say a violent incident took place there.

Forensics teams have not yet determined whether the blood belonged to one person or several, but police said whoever it belonged to would have been in medical distress.

As well, police have not confirmed whether marks examined by the forensic team on the outside of the Liknes's house were blood.

"I can tell you, though, that the forensic crime scenes unit did examine some marks on the side of the house and we're in the process of determining their relevance to the investigation," said Andrus. "There were marks on the side of the house that went for a distance along the side."

Still no suspect, motive

Over 200 tips have been received by police so far and investigators say they do have a number of theories as to what happened.

However, none of those are being shared with the public at this point.

"It's still a missing persons investigation," said Staff Sgt. Doug Andrus with the Calgary Police Service's Homicide Unit. "We're hopeful we'll find these individuals and family members alive."

There is still no motive in the case and police say they have no suspects they can name but identified someone they want to speak with.

A green Ford F-150 truck was spotted several times in the area on the evening the violent incident is believed to have occurred and police say they believe the driver of the truck may have information that could help them.

Kathryn Liknes, Nathan O'Brien and Alvin Liknes

Nathan O'Brien, centre, and his grandparents, Kathy and Alvin Liknes, have not been seen since Nathan's mom left the Likneses' home the night of June 29. (Calgary Police Service)