A vulnerable teenage boy could be heard crying out for his mother as he was beaten with iron bars by a masked gang, police have said.
The victim sustained a number of broken bones in the vicious attack in the early hours of June 10 in Derry's Gartan Square.
Police have issued a fresh appeal for information in a bid to catch those behind the "barbaric" attack.
They have also released a picture of one of the iron bars used in the assault.
Chief Inspector Ivor Morton said: "The attack, carried out by a number of masked assailants armed with iron bars, was a terrifying ordeal for the vulnerable 15-year-old boy who was heard to have cried out for his 'mammy' whilst the attack was taking place."
He added: "The young victim sustained a number of broken bones as a result of the assault.
"He is now recovering from his physical injuries, and I hope that he will also recover from the psychological impact of this incident."
Mr Morton said there can be "no place in our society for this type of barbarity".
"All right minded people must turn their backs on those responsible for such incidents and do the right thing by giving police any information they have about their activities," he added.
Anyone with information has been asked to contact detectives at Strand Road, using the non-emergency number 101 and quoting reference number 28 of 10/6/17.
Alternatively information can be passed anonymously through Crimestoppers on 0800 555 111.