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 shot dead outside his office on Friday in what police are treating as a terror attack.

Police shot dead his attacker, 15-year-old Farhad Jaber, at the scene.

Jaber, an Australian born in Iran and of Iraqi-Kurdish heritage, is reported to have shouted religious slogans as he killed Mr Cheng, a police accountant.

Australian Prime Minister Malcolm Turnbull has said the attack "appears to have been an act of terrorism".

Jaber 'must have been influenced'

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.

Caption: A 24-year-old man was arrested alongside a 22-year-old man at Merrylands on a separate matter, and charged with fraud

New South Wales Police Deputy Commissioner Catherine Burn said on Wednesday that police suspect Jaber did not act alone, and that some of those arrested were known to police.

"Time will clearly tell about what their associations may have been leading up to the events on Friday," she said.

She was questioned by reporters on why Jaber, who is not thought to have had a criminal history, was not on the authorities' radar.

"It's a reality of life we can't be everywhere with everybody at every single second of the day," she said, but added that police believe he must have come under "some influence, whether it was ideologically, religious or politically motivated".

Acting Deputy Commissioner of the Australian Federal Police Neil Gaughan said police were co-operating with Turkish officials to find Jabar's sister, thought to have flown to Istanbul not long before the attack.

Caption: The arrests were part of a joint investigation by the NSW Joint Counter Terrorism Team and State Crime Command Homicide Squad

But he said there was no evidence that she was involved in the attack.

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.

The BBC's Jon Donnison in Sydney says Friday's attack has shocked Australians, amid concerns about the radicalisation of young people.

Dozens of Australians are thought to be fighting with Islamic State (IS) militants in Iraq and Syria and the Australian government has introduced new national security laws in response.

Experts are worried about the effect of returnees - and on those who support them - on security.

In September last year, police in Melbourne shot a man they called a "known terror suspect" a day after IS called for Muslims to kill Australians.