It's Australia's deadliest mass shooting since 35 people were killed in Port Arthur, Tasmania in 1996.
"The anger towards them now is completely gone". I don't feel angry.
Mr Cockman was estranged from the late Ms Miles.
The bodies of their mother Katrina Miles, 35, and grandparents Peter and Cynda Miles were also found at the property.
"Peter didn't snap. He's thought this through".
"I will not put a time frame on that, but they will continue under the hard and complex circumstances", he told reporters alongside Mr McGowan.
"All the kids died peacefully in their beds". The [police officer] that went through, he said they looked all peaceful. He did a good job. He was like my best friend.
The family had all been shot by firearms licensed to grandfather Peter Miles, who was among the dead. "That's Peter and Cynda".
The tragedy is being treated as a murder-suicide and Western Australia Police Commissioner Chris Dawson told a press conference on Saturday that they are not looking for any suspects.
Aaron Cockman's former partner Katrina Miles, their 13-year-old daughter Taye and sons Rylan, 12, Arye, 10, and Kadyn, 8 were all found dead at a private property in Osmington on Friday (local time). "If it wasn't for him, I wouldn't have Katrina, I wouldn't have her kids".More news: Trump threatens to take away credentials of media outlets over negative stories
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West Australian Premier Mark McGowan has met with grieving residents of the Margaret River community.
Recent US mass shootings to make global headlines include the deaths of 17 people at a Florida high school in February, 58 people on the Las Vegas Strip in October 2017 and 49 people at an Orlando gay nightclub in 2016.
"Can I especially commiserate with those people who have endured the unendurable".
Cockman said that after feeling "so much anger" during legal struggles over custody of his children, such feelings had vanished.
"It was a farming property".
The West Australian government would await the coroner's inquiry, he said.
"I'm not going to put a timeframe on that", Dawson said.
Three firearms licensed to Mr Miles were found by investigators.
"I wish to strongly emphasise police do not believe any other person is involved with these crimes".
Police have yet to confirm which family member was the shooter but said they were not searching for any suspects.