Tulay's body has never been found
The case of a girl allegedly murdered by her own family is a "wake-up call" over so-called honour killings in the UK, the Old Bailey has been told.
Tulay Goren, 15, disappeared in 1999 after continuing a relationship with a man her family disapproved of.
Tulay's father Mehmet, 49, and uncles, Ali Goren, 56, and Cuma Goren, 42, all deny murdering the teenager.
In his closing speech, Jonathan Laidlaw QC said it was "shocking" to see feudal" crime in 21st Century Britain.
The prosecutor said: "How a father with the support of two of his brothers could take a knife to his own daughter, if that is what occurred, is something that is really quite impossible to understand.
"If the prosecution are right, to add to the sheer brutality of what these men did is the cold-blooded nature of this killing."
He continued: "If there are those in this country who believe we do not face similar problems as other countries where honour violence occurs, this case will be something of a wake-up call."
The court has been told that Tulay's relationship with her boyfriend, Halil Unal, would have been unacceptable to her family.
He is a Sunni Muslim while the Goren family was from the Alevi branch of the faith.
Tulay went missing from Woodford Green, north London, in January 1999. Her body has never been found.
The brothers have tried to shift the blame to each other during the course of the trial.
Ali and Mehmet Goren, both from Woodford Green, and Cuma Goren, from Walthamstow, also deny conspiring to murder Mr Unal between May 1998 and February 1999.
Mr Laidlaw said none had shown any remorse, adding: "All that is on their minds are the lies they have told in order to escape conviction - a conviction we suggest all three richly deserve."
The defence counsel is expected to sum up on Tuesday.