Ross Parker, 17, of Peterborough, Cambridgeshire, was walking home with his girlfriend when he was attacked by a gang of four Asians, Northampton Crown Court heard.
Stephen Coward QC, prosecuting, said the gang sprayed aerosol in his eyes and punched, kicked and hit the teenager with a panel-beater's hammer before they stabbed him.
Shaied Nazir, 21, Ziaraff Mahrad, 21, Ahmed Ali Awan, 22, and Sarfraz Ali, 25, of Peterborough, all deny murder.
Mr Nazir's younger brother Wyed, 18, said he found his brother and Awan at the kitchen sink cleaning blood from a hunting knife.
He said Mr Awan told him that they had been in a fight and that he had stabbed someone.
He said that both men had blood on their clothing as did the two other defendants who were in a shed at the brothers' Peterborough home.
Blood-stained
Mr Coward said that in a shed near one of the defendant's homes police found a bag containing blood-stained clothing.
They also found a hammer, which had edges matching the type of injuries Ross sustained, and a knife identical to the one with which he was stabbed.
Some of the clothing contained blood that matched Ross's DNA.
The case was adjourned until Friday.