A teenage boy arrested over an alleged terrorism plot in Sydney was convicted of possessing a firearm just four months ago.
The 15-year-old boy, who cannot be named for legal reasons, is one of five males arrested on Thursday over an alleged plot uncovered in December 2014 to attack an Australian Federal Police building.
The boy was arrested at his family home in Georges Hall on Thursday morning and is expected to appear in Parramatta Children's Court.
In December last year, just days before police uncovered the alleged plot, he featured in a photo holding a calibre .30-30 rifle in Chester Hill between 8pm and 9pm.
Police uncovered the photo during a raid on his home.
In August, he was convicted of three counts of possessing an unauthorised firearm and was directed to have ongoing counselling with psychologist and Muslim community advocate Hanan Dover.
He was given an 18-month good behaviour bond in Parramatta Children's Court.
Fairfax Media understands the boy was heavily radicalised and has been under close surveillance for more than a year.
Police have also been working with his school and family in an attempt to rehabilitate him.
On Thursday, he was charged with conspiracy to conduct an act in preparation for a terrorist act, an offence that has a maximum sentence of life in prison.
"It's disturbing that we're continuing to see teenage children in this environment," NSW Police Deputy Commissioner Catherine Burn said.
"We don't know how the 15-year-old has got to the point where we will allege he got," she said.
The boy is one of five people charged over the alleged plot to attack the Australian Federal Police building, contained in rough, handwritten notes found at the home of Sulayman Khalid in December 2014.
Mr Khalid, 20, who spoke about Islamic State on the SBS program Insight last year, was charged over the notes in December last year and remains in Goulburn jail's Supermax section awaiting trial, but he will be charged with a further offence on Thursday.
He was allegedly the main player in the partially formed plot to attack the police headquarters.
Fairfax Media understands two other prison inmates will be charged - one of whom is on remand for several firearms charges, including owning disassembled firearms, belts capable of carrying large quantities of ammunition and books about guerilla warfare and the history of combat.
The fifth person arrested on Thursday is Ibrahim Ghazzawy, 20, who was apprehended at his home in Raby and is expected to face Campbelltown Local Court on Thursday.
Three properties in Merrylands, Bankstown and Wiley Park were also raided but no arrests were made.
The group's access to firearms was particularly concerning, police told Fairfax Media.
Australian Federal Police Deputy Commissioner Michael Phelan said the five men arrested on Thursday were associates of the men targeted in sweeping Operation Appleby raids in September 2014.
"We absolutely deemed that they had the capability and the capacity and the intent to carry out an attack here in Australia," he said.
After arresting Mr Khalid in December 2014, police have spent a further 12 months examining the documents allegedly found in his home and, after conducting physical and electronic surveillance, laid the extra charges on Thursday.
Fairfax Media understands the plot was not well formed and mentioned an AFP building but the men did not yet know where the building was located or how they would go about attacking it.
"Last December there were a number of documents seized that related to the search warrants we undertook in western Sydney, and those documents ... clearly talked about a plan, and there were government buildings named in those plans, specifically the [Australian Federal Police] building," Deputy Commissioner Phelan said.
"As a result of putting all of that information together, working through those documents, putting physical and electronic surveillance together, we were able to build a case of conspiracy for five people involved in the preparation of these documents."
Since September last year, 11 people have been arrested in Sydney as part of Operation Appleby.