Police said Friday a truck was spotted in the area on Sunday night when the three were last seen.
It's described as a late 80s or early 90s green Ford F-150 in very good condition.
Nathan O'Brien and Alvin and Kathy Liknes were reported missing Monday and police believe their disappearance is suspicious.
The boy had been at his grandparents' on a sleepover after the couple held an estate sale at their home on the weekend.
Police have said there was evidence the three may have been forced from the house.
The Amber Alert remains in effect.
Police had been looking at closed-circuit video and have received hundreds of tips from the public.
"We have no suspects we can name, but ... we have information that's allowed us to move forward with this investigation,'' Staff Sgt. Doug Andrus said at a news conference Thursday.
Police aren't calling the driver of the truck a suspect but believe he may have more information about the three missing people.
Investigators spoke with about 75 people Thursday who attended the estate sale at the home.
It's believed more than 200 people were at the sale and police continue to ask anyone who may have attended to contact them.
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