Four people have been arrested in Sydney, Australia, in relation to the killing of police worker Curtis Cheng.

More than 200 officers swooped on several properties on October 7, police said, arresting males aged between 16 and 22. A fifth man was held over unrelated fraud charges.

Curtis Cheng, 58, was shot dead outside his office on October 2 in what police are treating as a terror attack.

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

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

Australian PM Malcolm Turnbull has said the attack “appears to have been an act of terrorism”.

According to Australian media, some of the properties raided on October 7 had also been raided by police last year, in a major anti-terror operation in Sydney.

New South Wales Police Deputy Commissioner Catherine Burn said that police suspect Farhad 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.

Catherine Burn was questioned by reporters on why Farhad 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 Farhad Jabar’s sister, thought to have flown to Istanbul not long before the attack.

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

Police said the raids were not connected to the arrest on October 6 of another teenage boy, who attended the same school as Farhad Jaber.

The 17-year-old boy was asked about alleged social media posts, but became threatening and intimidated officers.

Local media reported he had written Facebook posts defending Farhad Jaber’s actions, and made threats against another police station.

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