Five people, including a 15-year-old boy, were facing terrorism charges in Sydney Thursday as part of an ongoing investigation into an alleged plot to attack Australian government buildings last year.

The teenager and a 20-year-old man from western Sydney were arrested and charged with conspiracy to prepare for a terrorism act, said Michael Phelan, the Australian federal police's deputy commissioner of national security. Three other people, already jailed on other terror charges, would be charged with the same offense later Thursday, he said. All face a potential sentence of life in prison if convicted.

Sky News Australia identified the 20-year-old as Ibrahim Ghazzawy. The names of the other defendants were not immediately available. 

Officials said the arrests were not related to a new terror threat, but linked to a plot they uncovered a year ago that prompted a series of raids in Sydney in December 2014. Those arrested Thursday had been under police surveillance since then, which prompted the charges.

Phelan said the group "clearly talked about a plan and there were government buildings named in those plans, specifically the [Australian Federal Police] building ... The planning documents were evolving so specifically there's one mention of AFP. The details in those documents will come out eventually in court."

Phelan did not specify how close the alleged plotters were to carrying out an attack, saying only, "how well those plans were developed is a matter that will go before the court."

Police believe the plot, and another one that would have targeted a random member of the public in Sydney, were influenced by extremists overseas, Phelan said.

"A lot of the people we are dealing with — and emphasizing this is only a small group of people — are clearly radicalized to the point of talking about and acting out with violence. How they've become radicalized, we don't actually know," he said.

Police have conducted a steady stream of raids across the country since the government raised the nation's terrorism warning level in September 2014 in response to the domestic threat posed by supporters of ISIS.

The latest charges come two months after another teenager, 15-year-old Farhad Jabar, shot and killed New South Wales police accountant Curtis Cheng outside the force's headquarters. New South Wales Police Commissioner Catherine Burn said those charged were "associates" of those responsible for Cheng's death.

The Associated Press contributed to this report.

Click for more from Sky News Australia.