A deaf mute with a history of mental illness was arrested yesterday on suspicion of killing his 69-year-old mother, the latest in a string of domestic tragedies this year. Cheng Yan-oi was found in a pool of blood in her North Point flat with at least two cuts to her neck, police said. A bloodstained knife was found beside her. Her 49-year-old son, whose T-shirt had blood on it, had earlier alerted a security guard on the ground floor. The woman lived with her husband and two sons on the 19th floor of Kava Mansion in Fort Street. The family owns a snack food shop in the district. 'He used his finger to write 999 in his palm to show the guard,' a police source said. 'He then brought the guard to his unit, where his mother was found face-down in a pool of blood in the living room.' She was declared dead at the scene by paramedics. Officers arrested the son on suspicion of homicide. Police said he was taken to Eastern Hospital for a hand injury. Last night the suspect was still being detained for questioning and no one had been charged. Detectives from the Eastern District Crime Squad are investigating. Of the 16 homicides and suspected homicides committed between January and June, five were domestic killings, police said. Another case that shocked the city recently was the discovery of an unconscious 10-year-old boy trapped inside a leather suitcase in his family's home in Wan Chai in February. The child died two days later and his parents, who pleaded guilty, were jailed for up to two years yesterday for manslaughter. Others included a 43-year-old woman who was found stabbed to death in Tuen Mun in March. Her 22-year-old son was charged over the killing. Police figures show the number of domestic violence cases rose by 31 per cent to 719 in the first six months of this year, compared with the same period last year.