The teenager who fatally shot a New South Wales police force employee has been identified as Farhad Khalil Mohammad Jabar.

Key points: Gunman identified as Farhad Khalil Mohammad Jabar

Gunman identified as Farhad Khalil Mohammad Jabar Not known to police, appears to have acted alone

Not known to police, appears to have acted alone Turnbull condemns "cold-blooded murder"

A senior police source has told the ABC that Farhad attended a mosque shortly before the shooting.

The source said the 15-year-old was armed with a revolver and did not know the victim, Curtis Cheng.

Farhad killed Mr Cheng as he was leaving police headquarters at Parramatta in Sydney's west about 4:30pm Friday.

The offender then fired several more shots at officers as they emerged from the building to respond to the incident.

He was killed when the officers, who are special constables, returned fire.

The ABC understands Farhad's sister Shadi went missing on Thursday.

Her family told police she had taken all her belongings.

Gunman Farhad Khalil Mohammad Jabar, 15, was not known to police. ( Instagram )

It is understood she flew out of Australia on a Singapore Airlines flight bound for Istanbul, and may be attempting to reach Iraq or Syria.

Police have searched Farhad's North Parramatta home and confiscated computer equipment.

Senior law enforcement sources told the ABC it appeared the teenager had acted alone.

They said he had earlier gone to a mosque or prayer hall in Parramatta where, after prayer, he changed into a black robe.

"The people there went looking for him after prayer," one source said.

"There is a fair bit of information that he acted alone."

The source said the youth had been "carrying on" outside police headquarters for a few minutes before the shooting.

"He drew attention to himself to the extent some people caught it on their iPhones," they said.

The ABC was told three special constables responded quickly.

"One of them took the lead, he [the 15-year-old] was shooting at them and the lead special constable was shooting at him," one of the sources said.

Police are now trying to trace the ownership and history of the revolver used by the teenager.

The gunman had walked past a plain clothes female detective.

"She was wearing a business suit and she wasn't carrying a gun," one of the sources said.

"This poor bloke [the victim] was apparently the first one to walk out of the building – he had a connection to the police force – that was it."

Sorry, this video has expired WARNING: GRAPHIC CONTENT Video shows a person lying on the ground in front of the NSW police headquarters.

The ABC has been told the youth had never come to the attention of police.

"We don't know anything about him," the source said.

But it is understood a relative was known to law enforcement or intelligence agencies.

"[The relative] was a bit of a problem, he did come to the attention of police and counter-terrorism [authorities]," a source said.

What we know about the gunman 15-year-old boy

15-year-old boy Iraqi-Kurdish background

Iraqi-Kurdish background Born in Iran

Born in Iran Actions believed to be politically motivated and linked to terrorism

Actions believed to be politically motivated and linked to terrorism Believed to have been working alone

Believed to have been working alone Has family in Australia

Has family in Australia Resided in the Parramatta area

Resided in the Parramatta area Was not known to police and had no criminal history

One source confirmed the teenager was a Sunni Muslim who was born in Iran.

He said he was of Iraqi-Kurdish background and may have been a refugee.

"It is interesting he is a Kurd, the Kurds are among those bearing the brunt of ISIS, it doesn't make any sense," they said.

Police executed a search warrant on the teen's home on Friday night and seized "laptops and iPads" which they are now examining for clues as to the boy's motivation.

Security at police headquarters was recently reviewed and will now be reviewed again, the ABC has been told.

'Cold-blooded' crime 'doubly shocking': Turnbull

Prime Minister Malcolm Turnbull condemned the "cold-blooded murder, targeting the NSW Police Service".

"It was doubly shocking because it was perpetrated by a 15-year-old boy and it underlines the importance of families, communities, leaders being very aware of whether young people are becoming radicalised," Mr Turnbull said.

He urged the community to remember the Australian Muslim community would be especially appalled by the crime.

"We must not vilify or blame the entire Muslim community with the actions of what is, in truth, a very, very small percentage of violent extremist individuals," Mr Turnbull said.