Australian police on Thursday charged five people, including a teen, with planning a terror attack on a government building. Authorities said the arrests were not related to a new threat, but connected to a plot they had discovered about a year ago, which triggered a series of raids in Sydney in December 2014, the Associated Press (AP) reported.

The men were under police surveillance since officials found out about their plot last year, and three of them had already been imprisoned on separate terror charges.

Among those charged Thursday were a 15-year-old and a 20-year-old man from western Sydney, said Michael Phelan, the Australian federal police's deputy commissioner of national security, according to the AP. 

Authorities said that the men plotted to attack a government building in Sydney, likely a federal police building, Phelan reportedly said. He added that police were not sure about the teen’s role in the plot. Authorities believe that the plan, and another plot aimed at public in Sydney, were influenced by foreign extremists, Phelan reportedly 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," Phelan said, according to the AP.

Catherine Burn, New South Wales police deputy commissioner for special operations, said that it was “disturbing that we are continuing to see teenage children in this environment."

"We are charging a 15-year-old with a very, very serious offense. This is an offense that has a maximum of life imprisonment. This is concerning not only to us, in law enforcement, this should be concerning to everybody," Burn said, according to CNN.

One of the three jailed men was scheduled to appear at a Sydney local court Thursday afternoon, CNN reported. The remaining two had been denied bail and are set to appear in court on Wednesday, the Australian Federal Police reportedly said in a statement.