(CNN) -- A 15-year-old Australian girl returned from studying abroad to learn that her entire family had been bludgeoned to death in their sleep, in what police on Monday call a targeted attack.
The girl's parents, two young brothers and her aunt died of wounds to the head and upper body late Friday or early Saturday, at home in the Sydney suburb of North Epping, police said.
Police did not find signs of forced entry and nothing was stolen from the house. Authorities think the family was killed in a "personal" attack, a police spokeswoman said.
Media reports said the father ran a convenience store and had given evidence to police about an armored car robbery outside his store earlier this year.
"I don't believe they have reached anything conclusive about the motive," said the spokeswoman, who did not give her name, in keeping with department policy. "They are keeping an open mind."
The 15-year-old returned from a study trip in the Pacific Island territory of New Caledonia on Sunday.
"I can't imagine what this young girl is going through," homicide Commander Geoff Beresford told Fairfax Radio on Monday. "It is certainly going to be a long and caring process for us to deal with her."
The police department said it had set up a task force to investigate the deaths of the family, whose names were not officially released.
A family member called police Saturday morning after discovering the bodies of the 45-year-old man, his 43-year-old wife and his wife's 39-year-old sister-in-law.
The two boys who also were killed were 12 and 9.