Police arrested four people during a series of raids Wednesday in connection with the slaying of a civilian police worker, which officials have said they believe was linked to terrorism.

More than 200 officers swooped into homes in western Sydney and arrested the men, aged 16 to 22, as part of their investigation into the killing of Curtis Cheng, New South Wales police said. A fifth man was also arrested during the raids on unrelated fraud charges.

Cheng, a police finance worker, was shot by an Iranian teenager while leaving work in the western Sydney suburb of Parramatta last Friday. The killer, 15-year-old Farhad Jabar, was shot dead by police.

Police believe the killing was politically motivated and therefore linked to terrorism, though they say Jabar's specific motivations remain unclear and he has not been linked to any terror group. They also didn't know whether Cheng was personally targeted, or targeted more generally because he was a police employee.

Police don't know what the association might be between Jabar and the men who were arrested on Wednesday, and Deputy Commissioner Catherine Burn declined to say what part officials believe they played in last week's shooting beyond allegedly having knowledge of the attack. But police don't believe Jabar acted alone.

"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," Burn told reporters. "It's a very, very serious concern that in the heart of our community there is attack planning that is underway and that may have led to what we saw on Friday."

Some of the men arrested Wednesday were also investigated during a massive series of counterterrorism raids in Sydney last year, Burn said.

Jabar, who was born in Iran and lived with his family in the Parramatta area, was not on officials' radar before Cheng was shot. Burn acknowledged that police had no idea he was a threat, despite his alleged association with those investigated during last year's terror raids. That prompted questions about whether police should have been paying closer attention to him.

"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," Burn said.

Neil Gaughan, acting deputy commissioner of the Australian Federal Police, confirmed reports that Australian police were working with officials in Turkey to locate Jabar's sister, who is believed to have flown to Istanbul shortly before Cheng was killed. Gaughan said there is nothing to suggest she was involved in the attack, but police want to talk to her about what she may have known about her brother's plans.

Australia has been struggling to cope with a string of homegrown terrorism crimes involving teenagers. In September 2014, an 18-year-old was shot dead by police after stabbing two counterterrorism police officers in Melbourne. In April, several teens were arrested on suspicion of plotting an Islamic State group-inspired terrorist attack at a Veterans' Day ceremony. And in May, police arrested a 17-year-old in Melbourne and accused him of plotting to detonate three homemade pipe bombs.