SYDNEY — A teenager and four men were charged Thursday with planning a “terrorist” attack on an Australian government building, part of an ongoing investigation into the alleged plot, police said.

A 15-year-old and 20-year-old man from western Sydney were arrested and charged with “conspiracy to conduct an act in preparation for a terrorist act,” said Michael Phelan, the Australian federal police’s deputy commissioner of national security. Three other men, already jailed on separate terror charges, were later charged with the same offense, police 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 led to 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.