This green truck was seen in the Parkhill area the night a boy and his grandparents went missing.

Image Credit: Calgary Police Services

CALGARY - Calgary police investigating the disappearance of a five-year-old boy and his grandparents say there was a violent incident in the home where the three were last seen five days ago.

Homicide unit Staff-Sgt. Doug Andrus said police found forensic evidence at the grandparents' home, but wouldn't say what it was.

"Based on what we saw at the scene we believe there was a violent incident inside the residence," Andrus said at a news conference Friday.

"We've sent the exhibits away and we will have a better understanding of that once we get the results back from those exhibits, but based on the evidence, I would say that somebody would be in medical distress."

Nathan O'Brien and his grandparents Alvin and Kathy Liknes were reported missing Monday morning by the boy's mother.

The boy had been at his grandparents' on a sleepover after the couple held an estate sale at their home on the weekend.

Andrus said there were no signs of forced entry at the home and police still don't have a motive for what took place.

He said police are still treating the case as a missing persons investigation and the Amber Alert that was declared Monday remains in effect.

"I can't go into details about what we found in the residence," he said. "We are continuing to assess that information and we are still hopeful we will find these individuals alive."

Police are looking for the driver of a pickup that was noticed several times in the area on Sunday night when the three were last seen.

A picture of the truck captured on closed-circut video shows a late 80s or early 90s green Ford F-150 in very good condition.

Police aren't calling the driver of the truck a suspect, but believe they may have more information about the missing people.

Investigators put out a call to the more than 200 people they believe went to the sale at the Liknes' home over the weekend.

The pair were selling their things in perpetration for a move to Edmonton and then on to Mexico.

Andrus said officers spoke with about 75 people on Thursday.

Anyone with information can contact police through the non-emergency number 403-266-1234 or online at homocidetips@calgarypolice.ca.

Hundreds of leads and tips have been shared with investigators as of July 4.

Nathan O'Brien and his grandparents Kathy and Alivin Liknes were last seen June 29, 2014 following an estate sale. The Liknes planned to move to Edmonton.

Image Credit: RCMP HANDOUT

— The original story was replaced with a Canadian Press version containing more details.

News from © The Canadian Press, 2014