Two days after he killed 32 people in America's worst college shooting, it has emerged that the student gunman Cho Seung-Hui was known to the police and had previously been sent to a mental health unit over fears he was suicidal. The revelations are the latest about a 23 year old whose moods and behaviour had attracted concern from fellow students and teachers at Virginia Tech. Police have said that Cho had been the subject of two separate complaints from women eighteen months ago. None of this prevented his being sold a gun. Meanwhile tributes have been paid to the victims, including the heroic actions of Professor Liviu Librescu, who saved many of his students' lives.