A family of four who were found dead in their Sydney home on Monday may have died from an "airborne gas" deliberately introduced to the home in a suspected murder-suicide.
Police confirmed they are investigating whether a gas was filtered through the Davidson home, killing Maria Claudia Lutz, 43, Fernando Manrique, 44, and their two children, Elisa, 11, and Martin, 10.
It has been revealed both children had autism and went to St Lucy's Catholic Primary School in Wahroonga -- which is a school for children with disabilities. Mothers at the school called police on Monday when Lutz didn't arrived for canteen duty.
"Whilst the cause of death will be a matter for the Coroner to determine, police are looking at the possibility the family members died from the introduction of an air-borne gas," NSW Police said in a statement on Tuesday.
"It is early days in the investigation and investigators will need to wait for toxicology results and results of the post-mortems to determine cause of death."
Neighbours have reported seeing Mr Manrique on the roof with power tools days before the family were found dead in the home.
She always took the positive view of life, that the children were alive and she was alive, that's the kind of phrase she would use, you know, the children were having a few issues, but they're alive.Warren Hopley
On Tuesday Maria Claudia Lutz's friends, who described her husband as a private man, paid tribute to the "amazing mother" and "selfless person" at a memorial.
Karen Hickmott, who was one of Lutz's many friends at St Lucy's, told The 7:30 Report the mother-of-two never revealed any of her troubles and was "a fighter".
"She was always caring about everyone else, it didn't matter what was happening in her life, she was always most worried about everyone else," Ms Hickmott told the program.
Hickmott was joined by other mothers, teachers and students at the school on Tuesday as they tried to comprehend the death of their good friend.
Acting principal Warren Hopley said Lutz always took something positive from a situation.
"The children needed a lot of extra support and it wasn't easy for her but she was always very strong," Hopley said on the program.
"She always took the positive view of life, that the children were alive and she was alive, that's the kind of phrase she would use, you know, the children were having a few issues, but they're alive."
When police arrived at the home at about 11.20am on Monday, all four family members' bodies had no visible injuries. The family dog was also found dead.
The State Crime Command's Homicide Squad is assisting officers from the Northern Beaches Area Command in the investigations as forensic police continue to recover evidence from the home on Sir Thomas Mitchell Drive.
Police are urging anyone with information to contact Northern Beaches police or Crime Stoppers on 1800 333 000.