Four men were charged on Wednesday with plotting the Parramatta shooting and killing Sydney police accountant Curtis Cheng last October and are expected to face the Goulburn Local Court on Thursday through video link. One of the accused helped plan the shooting and assisted 15-year-old gunman Farhad Jabar’s sister to travel to Syria.

The other three gunmen, who were already charged with the murder, are now facing fresh charges.

Milad Atai, 20, a painter and an ISIS terrorist, helped Jabar’s sister to travel to Syria a day before the fatal shooting of October 2 at Sydney’s Parramatta police headquarters. He was arrested last month in Operation Appleby, Australia’s most extensive counter-terrorism investigation. However, he was not charged with the murder until Wednesday, the ABC reported.

Police allege that the suspects worked together with Jabar, who was shot dead after the attack by police.

The four accused, whose ages range from 18 to 23, conspired about the attack on Parramatta police headquarters seven months earlier. The 18-year-old has been accused of supplying guns to Jabar at the Parramatta mosque on the day of the shooting that killed Cheng.

Atai was already facing a charge of collecting funds for the terrorist organisation. All four men will face a possible life imprisonment for their offence. The other two are also facing charges of gun supply and membership to a terrorist organisation.

NSW Police Deputy Commissioner Catherine Burn said that the charges against the four men are now “very, very serious.” “We will be alleging that they were involved in a plot, which they were going to carry out, which was a terrorist attack,” The Australian quoted her as saying.

She added that though this is an ongoing investigation, according to the police these are the principal accused responsible for carrying out the attacks. “We have come across information that suggests to us they did come together in at least the day of and perhaps even the day before,” she said.