Twin two-year-old girls, another girl aged three and a half, and their mother and grandmother are the victims of what is feared to be the third mass killing in Western Australia this year.

The ages of the children and their relationship to the two adult women found dead at a Bedford home in Perth’s east were confirmed on Monday by Western Australian police.

“It is a tragic thing when incidents like this occur,” said an assistant commissioner, Paul Steel. “It does send a ripple through the community of Western Australia.”

The shocking discovery came after the shooting deaths of three adults and four children in Osmington in regional WA in May, and the deaths of a mother and her daughter, 15, and son, eight, at Ellenbrook in Perth’s north-east in July.

A man in his 20s turned himself to a regional police station on Sunday. Police then went to the Bedford home where they discovered the bodies about midday.

The house where the bodies were found. Photograph: Reuters

It is believed the man was known to the women and children, although police have not confirmed this. “The man currently assisting police remains in custody,” police said on Monday. “No charges have been laid at this time.”

Officers have not said how and when the women and children died.

Homicide detectives and specialist forensic police will continue to comb the Coode Street home for evidence. “Depending on the size of that crime scene that will take a period of time, and it could be a number of days,” Steel said.

• In Australia, the crisis support service Lifeline is on 13 11 14 and the national family violence counselling service is on 1800 737 732. In the UK, Samaritans can be contacted on 116 123 and the domestic violence helpline is on 0808 2000 247. Other international helplines can be found at www.befrienders.org