A home raided by heavily armed police officers as part the investigation into the death of police accountant Curtis Cheng was targeted several months ago by officers searching for firearms.

On Wednesday morning neighbours watched as heavily-armed police searched and  arrested Talal Alameddine, 22, and Rafat Alameddine, 24, outside their Merrylands home in a pre-dawn counter-terrorism raid.

Police are investigating the possibility the gun was sourced from a Middle Eastern crime gang before it was given to the radicalised high school student.

In February the same dual-level Merrylands home was searched by police after a suspected plot to create a potentially dangerous disturbance at a Sydney court complex.

As police raided the Alameddine home on Wednesday as part of the investigation into the shooting of police accountant Curtis Cheng, Talal was due to appear in court for possessing an unauthorised weapon in Merrylands.

The charge dates back to August 19.

Rafat had months earlier been found not guilty on two weapon related offences.

The raid on Wednesday came as police try to hunt down how an older-style handgun ended up in the hands of 15-year-old Farah Jabar, who shot dead Mr Cheng last Friday.

While police say Talal, 22, was arrested as part of the counter-terrorism raids rolled out across Sydney's west on Wednesday, Rafat was arrested on a warrant for unrelated fraud charges.

Talal was released from custody without charge hours later after the detention period on a police warrant expired.

Rafat, 24, returned to his family home just after midday but was turned away by police, who were still inside searching the property, and clashed with reporters as he left.

A friend of the family later told Fairfax Media he was with the boys last Friday and they hadn't been at Parramatta Mosque, where police believe Jabar was handed the gun.

"They are great boys, they are innocent," he said.

The friend said they had only been talking about how bad last Friday's shooting against an innocent man was in the days before the raid was carried out.

Fairfax Media understands the Alameddine household was targeted in a firearms raids in February after police received a tip-off about a potential disturbance to court hearings at a Sydney court complex.

On that occasion police were searching for "prohibited firearms". No one was charged.

The night prior, it is understood detectives from the State Crime Command received a tip-off that the Downing Centre Court, which was hearing a court case involving two men with lengthy criminal histories, could be disrupted.

Fairfax Media has been told police were left with no choice but to then throw immense police resources at the court complex to prevent any violence.

A resident who lives across the road from the Alameddines had been at home to witness both Wednesday's and February's raid.

"This is not the first time it has happened," he said.

"It happened (earlier this year) over something to do with a court case in the city."

Residents say police arrived outside the Merrylands home just before 6am on Wednesday and used a megaphone to call the occupants outside.

"They said 'residents of number 11 come out, your house is surrounded, you will not be harmed'," one resident said.

The man said the boys came out of the house one by one and were searched before being led down the street by police.

One female relative walked out of the house and clashed with reporters on the street, grabbing a journalist's microphone and pushing a camera away.