When detectives arrived at the crime scene on Wimbledon Common on July 15 1992, one thing puzzled them more than anything else: someone had placed a tiny, folded-up piece of paper on the forehead of the murder victim, Rachel Nickell. The paper had the pin number of Nickell's building society account written on it.
What sort of killer would stop to place a small piece of paper on his victim's forehead? The police kept this odd fragment of information from the press, but later passed it on to the FBI's criminal profilers.
According to former detective inspector Keith Pedder, who wrote a book about the still-unsolved murder, it was more than a year later that the truth emerged. It wasn't the killer, or indeed any adult, who had put the piece of paper on Nickell's face: it was Alex, her son, who was two when he watched her being repeatedly stabbed - 49 times in all.
After the killer had gone, and before anyone had stumbled upon the scene, Alex stayed close to his mother's body. He tried to get her to wake up; at some point he saw the piece of paper on the ground, and he put it on her forehead - perhaps, Pedder speculated, in an attempt to help her. But it was only a long time afterwards, when Alex had turned three but still kept on bringing up the murder, that his father called police in to re-interview the child and a small mystery, in the midst of what remains a much larger mystery, was unravelled.
Alex is 15 now, and lives with his father, André Hanscombe, in France. He is, according to the very few interviews Hanscombe has ever given, doing OK. But he and his father will probably have heard about the case of Abigail Witchalls, who was attacked in front of her 21-month-old son last week, and it will have brought back some dreadful memories. Nickell died, Witchalls survived - but the two cases have awful parallels: both women were alone with their toddler sons; both appear to have been stabbed by strangers in motiveless, frenzied attacks. Nickell and her son - by the little boy's account - were looking for blackberries on the common when it happened; Witchalls and her son were on their way back from a parent-and-toddler group on a wooded track in a Surrey village.
Witchalls has managed to tell police - communicating with blinks and by mouthing "yes" or "no" - that she fled from her attacker, pushing Joseph in his buggy, but the man caught up with her. He held a knife to Joseph's throat before grabbing her and stabbing her in the neck. The man then threw the buggy - with Joseph in it - down on top of her, before running off.
Yesterday the papers printed pictures of Joseph visiting his mother, who is paralysed and on a ventilator; he was photographed leaning out of his father's arms to touch her face. How will Joseph cope with what he has been through, and what lies before him? How will he cope if his mother can never pick him up again? Nickell's son has survived - but we have no idea what he has been through: survival isn't everything.
Dr Sean Perrin is one of a handful of people in this country who specialise in childhood trauma, and how to treat it. He works with children at the Institute of Psychiatry's child traumatic stress clinic at the Maudsley hospital in London. Some of the children are asylum seekers - mainly from sub-Saharan Africa, or Eastern Europe - others have witnessed domestic abuse, or been involved in road traffic accidents, or had surgery that has gone badly wrong.
Perrin's view is that the folk belief that children recover easily - that they're very robust and, with lots of TLC, will soon get over it - has a limited basis in reality. "People are a little bit too overconfident, too inclined to believe that the children get over this and bounce back, because the symptoms of post-traumatic stress disorder are internal to the child - they're a private phenomenon. You can suffer in silence, and to a great degree."
Children under three, who are not really talking much, will be - to a degree - insulated from the impact of the sort of severe trauma Joseph has undergone. "The more language you have, the richer the memory is," says Perrin. But this doesn't mean that Joseph won't show symptoms of being traumatised - irritability, whinging and tearfulness; reverting to habits he has long since given up, such as waking at night. And with children who aren't really talking properly yet, Perrin says, there's a double whammy: sometime, somewhere, someone is going to tell them what happened to them, and then they are going to have to live through it all again, on a verbal level. That is when the classic symptoms of post-traumatic stress disorder (PTSD) are most likely to start appearing: intrusive recollections, such as flashbacks and nightmares; a numbing-down of emotions, avoidance of anything related to the trauma, and withdrawal from the world; and physiological problems, such as headaches, stomachaches and an exaggerated startle reflex. It is the final group of physical symptoms that are easiest to identify in small children.
About a third of children who have some kind of severe one-off trauma develop PTSD, Perrin estimates. For children who witness something truly awful - such as seeing their mother stabbed in front of them - the incidence is probably twice that, even if they are as small as Joseph. If these children get the right help - letting sleeping dogs lie is not the answer here, Perrin says - then they can make full recoveries.
But getting the right help often isn't possible. Most parents would not know how to recognise PTSD in their children, and if they work out that something is wrong they take them to a GP; the GP then puts them on a waiting list for a very long time, and after that it is highly likely the child will see someone who does not know anything about PTSD in children - and the child will not get the right treatment. Unless they live in the catchment area for the Maudsley, that is, or get referred there or to Great Ormond Street, which offers a similar service.
Those who are lucky, though - and thousands every year are not - get two levels of treatment. First, doctors make sure that they have a proper routine and plenty of social support - Joseph is lucky that he is surrounded by a loving family, as Alex was too: it really does help in these cases - and then the doctors encourage the children to talk about their memories. The trick, Perrin says, is to subtly alter the way they remember the trauma. If a child has been involved in a terrible accident, for example, and thought that they were going to die, you remind them that they didn't die, so that the memory becomes: "I was in a terrible accident and I thought I was going to die, but I didn't." Slowly they learn that, as Perrin puts it, "the memory won't kill them".
The memories won't kill Joseph - just as they didn't kill Nickell's son. And we know that children who have survived even worse traumas - as well as the added stress of constantly seeing their mother's name in the papers - can find happiness and stability. Josie Russell, who saw her mother and sister murdered in a muddy field in Kent in 1996 - and who herself sustained the sort of injuries doctors normally only see in major traffic accidents - has gone on, despite enormous hardship, to pass eight GCSEs. She lives with her father in Wales, and is currently doing a design course at a college about 15 miles from home, and seems to be getting on with her life just fine.
But the effects of trauma, if a child doesn't learn to have "mastery over the anxiety", can have serious long-term consequences; they may end up drinking too much, or smoking too much, or may just be hugely vulnerable; easily knocked over by life. "The good news is that, for the most part, people do get better," says Perrin. "But it depends on the trauma, and also on how vulnerable the child is ... If you've had a really crappy life up to that point, you're not going to respond as well."