The Sydney family of four found dead in their home yesterday are believed to have been deliberately gassed to death with a poison that was released using devices throughout the house.
Mother Maria Claudia Lutz, 43, was found in one room with one of her children and her other child was found alone in a separate room.
Fernando Manrique's body was found in another room in the house.
Mr Manrique was spotted working on the roof of the house using power tools just days before the tragedy occurred. It is believed he was installing some sort of system to disperse the poison throughout the house.
Police have not yet found a note.
"There was a lot of pre-planning that went into this," a police source said.
A neighbour said she watched Mr Manrique, 44, working on the house with power tools just two days before his death.
According to the Drake Business Logistics website, where Mr Manrique worked, his duties included being "actively involved in re-engineering and company mergers, where he has used his expertise and engineering training to successfully bring operational and strategic goals together".
Ofik Thomassian, 72, lives directly across the street from where the family died.
"He was very friendly and she was very protective of those children," Ms Thomassian said.
"She would lead them one at a time into the car, lock the door, and return to the house for the next child."
Police confirmed they were looking at the possibility the family died as the result of "the introduction of an air-borne gas" and said they were waiting on toxicology results to determine the type of gas.
Investigators are also attempting to locate where the gas was purchased.
Tributes began to flow as neighbours mourned the loss of the loving family of four, including the couple's two autistic children Martin, 10, and Eliza, 11.
Their family dog was also found dead nearby.
One by one, heartbroken friends laid flowers outside the home and shed tears for "the innocent children" whose lives were stolen far too soon.
Neighbour Brooke Topschij was planning to celebrate her 18th birthday today but what was to be a joyous night out alongside friends has turned into a painful test of her strength.
The teenager has called on police not to delay releasing an official verdict on the deaths in order to give those who knew the family closure.
"It breaks my heart to think of how much pain the family was going through," she said.
"We just want to know for sure if what we all think happened is what happened.
"It will give us peace of mind to know that it was not a murder, but a decision."
Ms Topschij was accompanied by another neighbour as she placed flowers and a private note on the doorstep of the family home.
"She looked after them but I have to say she never looked happy.
"She always had a very serious look on her face. Thinking back now, she looked like she was in pain. She probably had been for a very long time."
Premier Mike Baird today expressed his shock over the tragedy.
"Like everyone across the state that sort of tragedy stops you in your tracks," Mr Baird told a press conference today.
"I can't imagine the circumstances but all I know - for their other family and friends - our hearts go out to them and our thoughts go out to them. Just tragic, tragic circumstances."
Nearly 24 hours on from the discovery of the bodies, forensic crews are still working inside.
The property has been cordoned off and police are cautioning the well-meaning public from getting too close.
It could be weeks or even longer until the narrative leading up to the family's death becomes complete.
Where to get help:
• In an emergency: call 111
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633, or text 234 (available 24/7) or firstname.lastname@example.org or live chat (between 7pm and 11pm) http://livechat.youthline.co.nz/mibew/chat?locale=en&style=youthline
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155 (weekdays 11am to 5pm)
• Samaritans 0800 726 666