Farhad Khalil Mohammad Jabar gunned down 58-year-old NSW police accountant Curtis Cheng in cold blood but the teenager wasn't on the radar of authorities as a potential terrorism threat.
NSW Police Deputy Commissioner Catherine Burn says investigators are still assessing the boy's motivation in carrying out the murder on Friday outside the Parramatta police headquarters.
"The 15-year-old, he was not a threat to us," she told reporters in Sydney on Wednesday.
"He was not a target for us.
"For 24 hours 7 days a week, people go and do certain things and it's a reality of life we can't be everywhere with everybody at every single second of the day."
Asked if Jabar should have been on the radar of police and counter-terrorism bodies, Ms Burn said "time will clearly tell".
Investigators are also examining whether he had connections with a group of males arrested in Sydney's western suburbs on Wednesday as part of a joint investigation by the NSW Police Force, State Crime Command Homicide Squad and the Australian Federal Police.
Jabar's sister is believed to have flown from Sydney to Istanbul the day before he carried out the attack.
The AFP is in contact with Turkish national police to try to establish her whereabouts.
The agency stressed this was not to suggest she was involved in any criminal activity in Australia.
"But obviously investigators are very keen to have a chat with her about what she knows about her brother's action," AFP Acting Deputy Commissioner Neil Gaughan said.
Ms Burn said investigators remained in the dark about how Jabar got hold of the gun he used to shoot Mr Cheng.
She did not rule out the possibility the group arrested on Wednesday could have supplied it.
"The supply of the firearm is a major part of our investigation and that is a line of inquiry we'll be undertaking with these people today," she said.
Ballistic testing of the gun is ongoing.
"The examination of the firearm is continuing, as is the examination of other pieces of evidence," she said.
"It's probably not that useful to go into too much detail but if there are those sort of forensic ballistic details, that will be a part of our investigation."
Authorities were doing all they could to keep people safe.
"We have been saying since September last year that a terrorist attack is likely and this is why our threat level was raised to high," Ms Burn said.
"It is just a sad, unfortunate reality of the environment that we are now in is that we are not necessarily able to be everywhere at all times.
Nevertheless, can I assure you, though, that our security agencies and our Commonwealth partners and all our jurisdictions, we are all working together to keep people as safe as we possibly can."
