Chief inspector Frankie Taylor made the comment at a press conference held in the Rath Mór centre where members of the dead man's family made an emotional appeal for anyone with information to come forward.

The detective had a clear message for the killers. "They know who they are and we know who they are. They need to come forward and speak to us," he said.

Speaking at the press conference, Mr. Sheils' uncle, Patsy Moore said the shooting was a "deliberate act."

"Emmett's name may have been Sheils but he was part of the Moore family. We fostered him when he was seven months-old and he has been a part of our family ever since. Today we are a family in mourning, we are in deep sorrow over the loss of Emmett. This is a terrible tragedy which has been visited upon our family and we are finding it difficult to come to terms with his murder.

"This was not a case of mistaken identity but a deliberate act and we are calling on anyone with any information to bring it forward so that those who carried this out this murder are held responsible," he said.

Mr. Moore also thanked the paramedics and medical staff who helped the 22 year-old as he was dying and thanked the community in Creggan for their response to the vigil which was held in protest at the shooting.

Maureen Wilkinson, an aunt of Mr. Sheils, called for no retaliation for the murder. "We don't want any other family in Derry to have to go through what we are going through," she said. She also thanked the PSNI for supporting the family since the shooting.

Chief Inspector Taylor said the PSNI had received an "overwhelming response" from the community in Creggan.

"We have carried out five searches and in response to that two people have come to the police station and have subsequently been arrested. There are more individuals we need to talk to and my message to them is clear; come forward," he said.

The detective also said he believes more than one of the gang who shot Mr Sheils was armed. "Our inquiries at the moment suggest there was five or maybe six individuals involved and that more than one of them was armed," he said.