Airdrie searchers use boat

Photo: The Canadian Press. All rights reserved.

Police investigators comb a hay field north of Airdrie, Alta., Saturday, July 5, 2014, looking for clues to the disappearance of three people. THE CANADIAN PRESS/Jeff McIntosh

Investigators brought in a dog and a police motorboat on Sunday as they shifted their search for a five-year-old Calgary boy and his grandparents to a field and a slough north of an acreage.

The area police were focusing on Sunday is about a kilometre north of the acreage near this Calgary bedroom community that police combed on Saturday.

Yellow police tape blocked off an adjacent road and a half dozen officers and a dog walked through a field to the edge of a large slough. Moments later a Calgary police boat was towed into continue the search while a police helicopter searched overhead.

Earlier in the day police revealed they had released an individual taken into custody at the acreage Friday night.

Calgary police said he remains a "person of interest" and several police units will continue to search a residence connected to him on the outskirts of Airdrie.

Investigators say that, given the size of the property, the search is likely to continue throughout Sunday and into Monday as they search for clues in the disappearance a week ago of Nathan O'Brien and his grandparents — Alvin and Kathy Liknes.

Acting on a tip police arrived at the property late Friday night and found a green Ford F-150 truck similar to one spotted in a video taken near the Liknes home.

Police say they are trying determine if the truck, the property, or the man taken in for questioning, are linked to the investigation.

Nathan and his grandparents disappeared on Monday after the couple held an estate sale, and police have said there was evidence of a violent incident in the Liknes's home.

Police say they want to hear from anyone who has any information regarding the whereabouts of the missing people or recognizes the pickup that was captured on video in the area of the grandparents' home.

Investigators say an Amber Alert remains in place and is continuing to provide police with tips from the public.