Police in Australia have arrested four people during a series of raids in connection with the murder of a civilian police worker that is believed to be terror-linked.
More than 200 New South Wales Police officers swooped on homes in western Sydney and arrested the men, aged 16 to 22, during their investigation into the killing of Curtis Cheng.
A fifth man was arrested during the raids on unrelated fraud charges.
Mr Cheng, a police finance worker, was shot by an Iranian teenager while leaving work in the western Sydney suburb of Parramatta last week. The killer, 15-year-old Farhad Jabar, was shot dead by police.
Police said on Saturday that they believed the killing was politically motivated and therefore linked to terrorism, though they said Jabar's specific motivations were unclear, including whether he was working alone or linked to a specific group.
They did not know whether Mr Cheng was targeted personally or more generally because he was a police employee.
Police do not 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. Police, however, do not 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," Ms Burn said.
"It's a very, very serious concern that in the heart of our community there is attack planning that is under way and that may have led to what we saw on Friday."
Some of the men arrested were also investigated during a massive series of counter-terrorism raids in Sydney last year, Ms Burn said.
Jabar, who was born in Iran and lived with his family in the Parramatta area, was not on officials' radar before Mr Cheng was shot.
Ms 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, seven 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," she 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 Mr Cheng was killed.
Mr Gaughan said there was 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 home-grown terrorism crimes involving teenagers.
In September 2014, an 18-year-old was shot dead by police after stabbing two counter-terrorism 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 that would have targeted police officers.
And in May, police arrested a 17-year-old in Melbourne and accused him of plotting to detonate three home-made pipe bombs.