Police used a helicopter and dogs to scour a forested area near the Coeur d'Alene, Idaho, home for any signs of Shasta Groene, 8, and her brother, Dylan, 9. Authorities said they had no clues as to their whereabouts. The FBI is also assisting in the search.
"Our main concern right now is the two children we cannot find," said Sheriff Rocky Watson.
Late Tuesday night, Sheriff's Captain Ben Wolfinger said that investigators are also looking for Robert Roy Lutner of Hayden, Idaho. Police say he is not a suspect, but because he is believed to have been at the family's home on Sunday, he is considered to be a "person of interest" and investigators would like to speak with him.
Lutner is reportedly driving a 1975 Silver Ford pickup with Idaho license plates K161057 or a 1990 white Toyota pickup with Idaho plates K308811.
The victims in the house are identified as Brenda Groene, 40, mother of the missing children; Slade Groene, 13, the children's brother; and Mark McKenzie, 37, of Coeur d'Alene.
Sheriff Watson said he did not know the relationship between McKenzie and the others, but news sources have reported he was the mother's boyfriend.
The Kootenai County Sheriff's office, which issued the Amber Alert, says it has no idea who took the children, reports CBS' Teri Okita.
"We don't have any leads of abduction, of direction of travel, or people of interests ... so we're just doing whatever we can," Watson said.