Former school principal Nathaniel Train had not been seen by his family since December 2021 but had remained in contact with them until a few weeks before the deadly siege in Queensland.
His sudden silence in October prompted a missing person report to Walgett Police.
NSW Police issued a public alert to try to find the 46-year-old due to concerns for his welfare.
"Nathaniel Train was reported missing to Walgett Police and the investigation around the circumstances of the horrible incident will be led by Queensland Police," NSW Police Commissioner Karen Webb said.
"Those officers were responding to a routine job, they were ambushed and they were executed.
"Our sympathies go to the Queensland Police Service, the families of the fallen officers and their friends."
Train left Walgett in August 2021 after he suffered a cardiac arrest while working as the executive principal at Walgett Community College School.
He had been in the role more than a year after teaching at schools across regional Queensland.
In an email sent in February 2022, he told a staff member at the NSW Department of Education he had recently spent time out in the bush and earlier this year had come off prescribed medications.
He went on to say his recovery was "going well".
A troubled history
Train's last teaching post was in Walgett and the NSW Department of Education confirmed he was employed until March 2022.
"One of the deceased was a former NSW Education employee who had not been working at a NSW school since August 2021," the department said in a statement.
"He officially left our employment in March this year."
Train expressed concerns about the educational policy he claimed to have observed at the school and spoke to the ABC in May 2022 about them.
He sent several emails to the department during his time and made a raft of allegations about teaching and discipline issues at the school.
Train said this triggered a breakdown in his relationship with the education department and alleged he was subsequently targeted by executives for raising those concerns.
Train said while he finished at the school after he suffered a cardiac arrest, he would not have stayed because of his refusal to get a COVID-19 vaccination.
The NSW Education Minister Sarah Mitchell said she was aware of the allegations raised by Train.
"There had been allegations raised previously with me in the parliament and also through our budget estimates processes in relation to some of the claims that had been made," Ms Mitchell said.
"We had department witnesses provide evidence, that's all publicly available, and none of those claims were able to be substantiated."
She said support was being offered to the Walgett school community and the education department would work with Queensland authorities as part of their investigation into the deadly shooting.
"We will provide any relevant information to the Queensland authorities as they investigate this matter," she said.
"It's horrifying and the Walgett school community as well, I can't even begin to imagine how they're dealing with this."
NSW Premier Dominic Perrottet expressed his condolences for the victims of the deadly shooting.
"On behalf of everybody across the state of NSW, pass on our thoughts and prayers to the families of those fallen police officers in Queensland who died doing their job," Mr Perrottet said.
The sails of the Sydney Opera House will be lit blue on Wednesday night in tribute to the police officers who were killed.
"We will be lighting up the Opera House to recognise their service to our great country," he said.
Community in shock
Parents of Nathaniel Train's former pupils at the Walgett primary school were reeling after learning he was involved in the murder of three people, including two police officers, in Queensland.
The 46-year-old moved to the town in mid-2020.
The school had struggled with truancy and staff retention for years and Train was considered by some parents to have made progress in addressing some of these issues.
"I think everyone will be struggling to comprehend what's happened because who would suspect a principal to do something along those lines?" parent Katrina Marshall said.
"It's a little bit confronting for a parent and, regardless of the circumstances, you're still shaken.
"We trust our kids with school teachers."
Walgett preschool staff member Donna Thurston said the entire town was in shock.
"It's scary — it's a small community, so everybody knows everybody," she said.
