Five people were facing terrorism charges on Thursday as part of an ongoing investigation into an alleged plot to attack a government building in Australia, police said.

A 15-year-old and 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.

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. That plot involved a plan to target a government building in Sydney, possibly a federal police building, Phelan said. Those arrested Thursday had been under police surveillance since then, which prompted the charges.

Phelan was vague on details of the alleged plot, and said police weren't sure exactly what role the teen may have played in the planning, what the specific target would have been or how the attack was to be carried out. 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 the Islamic State group.