An Australian man is accused of shooting his own two children to death and then turning the gun on himself, the Sydney Morning Herald reports.

Authorities say the victims, 13-year-old Jennifer Edwards and 15-year-old Jack Edwards, were gunned down by their 67-year-old father, John Edwards, in Sydney on Thursday.

The kids had been at the center of a lengthy custody battle with Edwards’ 36-year-old estranged wife, Olga.

Brett McFadden, acting assistant commissioner, said the “heinous crime” was “premeditated and planned.”

The newspaper reports that Edwards had another daughter and son who were born in the 1980s from an earlier relationship and who lived nearby. They are reportedly around the same age as Olga.

Edwards is said to have written letters to his former wives and family members that were to be opened after he died.

Investigators believe Edwards may have been planning the slayings for as long as a year, in part based on evidence from the letters.

Citing a confidential source, the Daily Telegraph reports the letters “would have raised concern,” although the exact contents of the writings have yet to be disclosed.

Information has also emerged about tension amongst family members. The Daily Mail reports that son Jack had told friends his dad was abusive and didn’t want anyone to know where they lived.

Olga also reportedly altered her appearance in the weeks preceding the death, in which she cut her long hair short and colored it from dark to blonde.

“There was always something really sad about her, but she never gave anything away,” said Michael Glenellis, the owner of a coffee shop located near Olga’s office.

The day before the shooting, Edwards gathered his guns from a gun club. He then arrived at the family home after the kids returned from school – but before Olga got back from work – and began shooting rounds of bullets.

Edwards then went to his own residence and shot himself. His body was discovered on Friday.

Multiple people who knew Edwards described him as a callous and uncaring man.

One neighbor said he was “known in the street as the grumpy guy with the border collie.”

Another person said Edwards was unhappy, harsh and never spoke kindly of his children. The source remembered an episode when Edwards grabbed his son Jack by the neck and pinned him to a wall.

“Poor Jack probably copped the brunt of it,” the source said.

McFadden, the acting assistant commissioner, said law enforcement had previous knowledge of Edwards but that he “did not have an extensive history.”

The Daily Mail reports that some gun clubs rejected Edwards from their membership because of “serious concerns” about him.

The principal of the school that Jack attended addressed the tragedy in a letter to parents, obtained by the Daily Mail.

“One of our Year 10 students died suddenly and tragically yesterday evening,” Principal Ross Warren wrote in the letter sent on Friday. “Our thoughts and heartfelt sympathy are with the student’s family and friends.

“I am bringing this incident to your attention both for information and so that you may be aware of your child’s concerns. It is important to try to include in your discussions positive ways of managing this sad situation.”