New Delhi, June 9 -- The video was already on phones across Northern Ireland before the Police Service of Northern Ireland had finished processing the scene. It showed a man straddling a victim on the pavement of Kinnaird Avenue in north Belfast, slashing at his head and neck with a kitchen knife while bystanders tried to drag him away. By morning, it had become a political rupture.

he is Sudanese. He had been granted leave to remain in the United Kingdom after arriving in Northern Ireland from Dublin, according to Assistant Chief Constable Ryan Henderson. No other suspects are being sought.

The victim, a man in his 40s, was taken to a Belfast hospital in serious condition with significant injuries to his face, eyes, neck and back. Hend...