Police rattled by ‘shocking’ crime scene in abandoned house where Sydney teen was sexually assaulted, bludgeoned to death

The abandoned house where a Sydney teen was sexually assaulted and beaten to death was a "shocking" crime scene, police have said.

Mahmoud Hrouk, 16, was found dead in Fairfield East by relatives on May 17 after disappearing the night before.

He had been fatally bludgeoned and police have now said he was also "brutally" sexually assaulted prior to death.

"That sexual assault was part of the reason he died," Detective Inspector Grant Taylor told reporters today.

"What remains a great mystery to us is why anybody would want to brutally sexually assault this young man and then bludgeon him to death."

Det Insp Taylor said somebody in the community had vital information about the murder and called on them to come forward.

Mahmoud was found by his parents as they searched for him after he vanished on May 16.

He was speaking to his mother over the phone at a McDonalds in Villawood at about 9.40pm when the call cut off without warning.

He had been having a burger with a friend.

Police have ruled out any organised crime links and have said the crime was "opportunistic", not planned.

Mahmoud's body was reportedly unrecognisable and police believe his attacker or attackers may have fled with his bloodied clothes including pants, underpants and running shoes.

The house he was found in was abandoned and had been used as a teen hangout in the weeks leading up to his death.

It is believed the assault and beating all took place within the house.

Det Insp Taylor said it appeared the attack had been carried out by just one person.

Anybody with information about Mahmoud's death is urged to contact CrimeStoppers on 1800 333 000.