Australian counterterrorism police arrested four people Wednesday during raids in connection with the fatal shooting of a civilian member of Sydney police by an Iranian teenager. Curtis Cheng was shot dead Friday by 15-year-old Farhad Jabar in what the police suspect to be a terrorism-linked incident.

New South Wales (NSW) police said that over 200 officers raided several properties in Sydney and arrested four men aged between 16 and 22. A fifth man was also reportedly arrested during the raids on unrelated fraud charges.

"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," Deputy Commissioner Catherine Burn told reporters Wednesday, according to the Associated Press (AP). "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."

Burn added that the connection between the arrested men and Jabar was unclear, but authorities don’t believe the gunman acted alone.

Jabar opened fire outside the NSW state police headquarters on Charles Street in the business district of Parramatta. Cheng was shot with a handgun at close range as he left work, authorities reportedly said. The Iran-born teen was shot and killed by responding officers.

"What we would suggest and we suspect is that there was some influence, whether it was ideologically, religious or politically motivated, that determined and influenced the 15-year-old to go and commit this horrendous act of violence,” Burn said, according to Reuters

Authorities also reportedly said that the teenager has not been linked to any terror group. Burn said that Jabar was not under the radar of authorities, and was not seen as a threat, but some of the men arrested Wednesday were investigated during a series of counterterrorism raids in Sydney last year.

Investigators are also reportedly seeking help from Turkish police to search for the shooter’s sister, who is believed to have flown to Istanbul shortly before Cheng was killed Friday. Neil Gaughan, acting deputy commissioner of the Australian Federal Police, reportedly said that there was nothing to suggest that she was linked to the attack, but police wanted to question her as part of the investigation.

Meanwhile, a report from the Daily Telegraph stated, citing police, that Jabar obtained the weapon, which was used to kill Cheng, from a Middle Eastern crime gang. Police reportedly suspect that the weapon was given to him at the Parramatta mosque.

The report also alleged, citing sources, that a fellow student at Jabar’s school was a supporter of the Islamic State group, also known as ISIS.

Australia has been on heightened alert after last year’s attack in Sydney where two people were killed after a lone gunman, Man Haron Monis, held 18 people hostage at a cafe, resulting in a 17-hour siege. 

The country has also stepped up counterterrorism efforts in recent months over a string of homegrown terrorism crimes involving teenagers. Hundreds of Australian nationals are supporting ISIS and dozens have traveled to Iraq and Syria to join the Sunni militant group in its fight against the West.