Police have confirmed that a violent incident took place in the Calgary home where a five-year-old boy and his grandparents went missing.

Staff Sgt. Doug Andrus said Friday that police found forensic evidence inside the southwest Calgary home, but didn't detail what the evidence was.

"We remain hopeful that we'll find them alive," Andrus said during a news conference.

Andrus said forensic experts are analyzing the evidence to determine the nature of the crime and who was involved. However, he said based on the evidence, police believe someone was in “medical distress.”

Nathan O'Brien and his grandparents, Alvin and Kathy Liknes, have been missing since Monday. Nathan had been staying with his grandparents at their southwest Calgary home over the weekend. When his mother went to the house on Monday morning, none of them were there.

The grandparents were in the process of moving out of their home and held an estate sale the weekend they disappeared.

Police announced Friday that they are looking for the driver of a green Ford F-150 pickup that was noticed several times in the area on Sunday night when the three were last seen.

Andrus said the driver of the truck is a "person of interest" who may have information about what took place before the trio went missing.

He said an amber alert remains in effect, and police have received more than 200 tips from throughout the country.

Investigators are appealing to anyone who attended the estate sale over the weekend to contact police.