Four people have been detained in Sydney, Australia in relation to the killing of police worker Curtis Cheng.
More than 200 officers swooped on several properties early on Wednesday, police said, arresting males aged between 16 and 22. A fifth man was held over unrelated fraud charges.
Mr Cheng, 58, was killed outside a police station on Friday in what police say was a terror-linked attack.
His attacker, schoolboy Farhad Jaber, 15, was shot dead at the scene.
According to Australian media, some of the properties raided on Wednesday had also been raided by police last year, in a major anti-terror operation in Sydney.
New South Wales Police Deputy Commissioner Catherine Burn faced fierce questioning from reporters in Sydney as to how Jaber failed to be on the radar of authorities, and was not seen as a threat.
"It's difficult because we don't really know the motivation of the 15-year-old," she said.
She said police believe Jaber must have come under "some influence, whether it was ideologically, religious or politically motivated" that made him "commit this horrendous act of violence".
"Let me say that today's operation is a clear indication of our determination to actually find out who murdered Curtis Cheng and to take all necessary action that we possibly can."
Police said the raids were not connected to the arrest on Tuesday of another teenage boy, who attended the same school as Jaber.
The boy, 17, was asked about alleged social media posts, but became threatening and intimidated officers.
Local media reported he had written Facebook posts defending Jaber's actions, and made threats against another police station.
Australia has stepped up counter-terrorism efforts in recent months in the face of what officials say is a growing security threat from radical Islamists.
Dozens of Australian nationals are thought to be fighting for Islamic State (IS) militants in Iraq and Syria.
Experts are worried about the effect of returnees - and on those who support them - on security.
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