A Sydney teenager was the victim of a brutal sexual assault before he was bludgeoned to death in what senior detectives described as one of the most "gut-wrenching" cases they have worked on.

Police believe that Mahmoud Hrouk, 16, was killed by an opportunistic attacker or attackers, who held him captive in an abandoned house at Fairfield East nearly two months ago.

"It's gut-wrenching": police are still hunting for Mahmoud Hrouk's killer. Credit:Facebook

His death was not believed to be gang-related or linked to organised crime, and police were shocked that such a "model child" was killed in such a horrific way.

"He was a model child dedicated to his family, and hard-working," Detective Sergeant Christian Olivares​ said.