Nina Wang's brother, who was attacked by a gang of four men on Thursday night, was last night discussing with detectives whether he should be placed under police protection, sources said. The move came after Kung Yan-sum, 61, a practising doctor, was discharged from Princess Margaret Hospital at around noon after receiving treatment. Dr Kung will be 62 next Thursday. 'The attack was an apparent ambush but the motive remains unknown,' said Senior Superintendent Chan Kwok-keung of New Territories South regional crime unit. 'It appears the assailants did not have time to make their next move and had to flee as a security guard heard shouting from the victim and went to check. 'We will look into all possible clues and information relating to the case.' He said the victim had not received any threatening calls or had any disputes before Thursday's attack. Yesterday, a team of plain-clothes officers returned to the alley in Hong Kong Garden in Castle Peak Road, Tsing Lung Tau near Sham Tseng, where Dr Kung was attacked. It is understood that investigators collected some cigarette butts from the alley and seized tapes from surveillance cameras at the private housing estate for investigation. Officers said Dr Kung usually walked through an alley with his dog and went to a nearby car park to pick up his car after work. His clinic is in the Hong Kong Garden commercial centre. On Thursday, Dr Kung and his dog were waylaid by the gang shortly after he entered the 100-metre-long alley at about 7.30pm. Police said one of the assailants was carrying an unknown hard object. During the attack, Dr Kung fell to the ground, was punched and kicked and also hit once in the face with a hard object, according to police. By last night, no arrests had been made. Police said Dr Kung's dog - a Chow that had tried to defend his owner - was beaten with the hard object and seriously injured. One of the four assailants was described as about 40 years old, 1.7 metres tall, and was wearing a dark cap and vest.