The sister of a man accused of slashing a woman to death and stabbing another in a Sydney CBD rampage has apologised for his alleged "despicable" actions.
Investigators are trawling through Mert Ney's electronic devices, social media accounts and family home as they try to determine why the 20-year-old sought to kill and injure innocent people on Tuesday.
Ney allegedly killed 24-year-old Michaela Dunn in a Clarence Street unit before stabbing 41-year-old Linda Bo at the Hotel CBD, walking down York Street with a knife and being restrained by bystanders.
Police believe Ney attended the Clarence Street apartment for "the purposes of prostitution". Ms Dunn was later found at the unit with a laceration to her neck and was pronounced dead at the scene.
Her mother told Nine she was a "beautiful girl from a beautiful family" and "very much loved".
The 24-year-old was a former Rosebank College student and studied at the University of Notre Dame with a friend describing her as a "true delight".
Ney's sister told reporters outside the family's Marayong home in western Sydney on Wednesday that Ney had been domestically violent in the lead-up to the incident and suffered several mental illnesses.
She said she didn't want to go near him and described his alleged actions as "despicable".
"All the words I can say won't ever bring her back. I want to say I'm really sorry, she was defenceless," said Ney's sister, who asked not to be named.
Police say Ney was carrying a USB stick with information about recent mass casualties in New Zealand and the United States, and could be heard on video footage shouting "Allahu akbar".
But his sister didn't believe he subscribed to extremist ideology.
"All this stuff about extremism, white supremacy, stuff like that, that's news to me ... it was using religion as an excuse," she said.
Ney remains under police guard in hospital and is yet to be interviewed by police about the incident that brought the city's streets to a standstill.
NSW Police Commissioner Mick Fuller said the rampage had not yet been classed as a terrorist incident and the "lone actor" - who is likely to be charged later on Wednesday - had no known links to terrorist organisations.
Mr Fuller also said Ney had recently self-admitted to a mental health facility after experiencing problems, and had been listed as missing by family.
The 20-year-old was captured on CCTV arriving at the Clarence Street apartment building at 1.30pm and leaving about 20 minutes later, before allegedly stabbing Ms Bo in the shoulder.
AAP understands Ms Bo is an employee at shipping company Cosco, which has an office nearby, and is in a stable condition in hospital.
"You would say with some confidence that he went on the street to kill more people, and the fact she's alive is a miracle," Mr Fuller said.
He then walked down York Street, armed with a large kitchen knife, before the bystanders' efforts to restrain him with chairs and a milk crate.
Ney has undergone surgery at Royal Prince Alfred Hospital for a cut to his knee. He is expected to be charged with murder, as well as serious assault.
The commissioner again praised the members of the public who helped restrain the 20-year-old, labelling them "true heroes".
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