Where is the tipping point in suspicion?
So often a murderer turns out to be someone who had regularly aroused suspicion before. But when should we act on our suspicions and report someone?
Every time a murder case finishes with a conviction, the media is finally allowed to reveal all the information they have gathered about the culprit.
Often a picture emerges of an unstable person, an "oddball", a fantasist, someone with a penchant for violent pornography, or who flew into violent rages. This was the case for Mark Bridger but it could have been any one of dozens of other murderers.
People always ask themselves whether they would have spotted that someone was more than just an "oddball"? Would they have acted on suspicions and reported them to someone in authority? Do they know someone now who they find unsettling?
During the trial of Bridger, it emerged he was a fantasist who claimed to be trained by the SAS despite having no military record. He would regularly wear camouflage and show off his knives.
He had a string of different relationships, six children by four different women and beat his girlfriends.
Everyone wants to stop serious crimes happening but can a pattern of behaviour like Bridger's lead to the authorities being able to act decisively? What is the role of ordinary people?
No-one wants to be ridiculed for reporting exaggerated fears. Or for making life difficult for somebody who is eccentric but otherwise innocent. At the same time, we sometimes feel we have a duty to do something.
After the Jamie Bulger murder 20 years ago, the tabloid newspapers described the people who saw his killers taking him on a lengthy walk - but who failed to act - as the "Liverpool 38". The implication was that they had failed in some way by being suspicious but doing nothing about it.
But on the more everyday level, how many of us have seen something suspicious at a neighbouring house and not done anything?
Society does need people to be aware of their neighbours, says Roy Rudham, chair of the UK Neighbourhood Watch Trust.
"You've got to get to know your neighbours and their vehicles and anyone suspicious in the road." People who say they've lived next to someone and don't know their name are foolish, he says.
"If you've got the slightest suspicion tell the police. Let the police prioritise it on the reports coming in."
It's the kind of spirit that was invoked after the murder of a soldier in Woolwich.
Dame Stella Rimington, former head of MI5, encouraged people who suspect their neighbours to inform the security services.
"The community has the responsibility to act as the eyes and ears, as they did during the war … where there were all these posters up saying the walls have ears and the enemy is everywhere," she said.
With both the neighbourhood oddball and a worrying extremist, the question is the same - where is the line past which I should report someone?
Criminologist David Wilson says it's impossible to pinpoint when oddball behaviour turns into something worth reporting: "There is no tipping point."
People may try to be wise after the event, and suggest they were always weird, he says. But small communities, like the one where Bridger lived, are good at accommodating people seen as odd.
In the rare event that a tragedy happens, a community seeks to make sense of what's happened.
"You tend to get one of two reactions: 'We always knew he was odd', or 'I'm amazed, I used to see him wash his car every Sunday, he was very ordinary'."
But where do you draw the line between an oddball or eccentric and a dangerous criminal or terrorist? The police do not have an answer.
A spokeswoman for the Metropolitan Police says it is difficult to give a line at which strange behaviour becomes potentially dangerous: "It's not quantifiable. It depends on the context." But she adds that the police rely on the public to come forward with information.
There are of course dangers with encouraging suspicion.
The case of Chris Jefferies shows how innuendo and gossip can lead to unjust accusations. He became the prime suspect in the murder of Jo Yeates after "sources" suggested he was "weird".
"Joanna Yeates murder suspect Chris Jefferies was last night branded a creepy oddball by ex-pupils, a teaching colleague and neighbours," the Sun reported. The Daily Mirror falsely branded him a "peeping Tom" on its front page.
But he was merely the victim's landlord and not a murderer. The real killer turned out to be Vincent Tabak. But the media speculation put an innocent man through an ordeal. "My identity had been violated," he later told the Financial Times. He later received substantial libel damages from the Sun, the Daily Mirror, the Sunday Mirror, the Daily Mail, the Daily Record, the Daily Express, the Daily Star and the Scotsman.
Peter Jones, co-director of the Social Issues Research Centre, says it's ridiculous to encourage the public to be more suspicious.
"If we rushed around ringing the police every time we think someone not quite right, the police probably wouldn't welcome it." Everyone knows eccentrics and people considered "odd". You may drink with them in the pub. Most of them do no harm, they just go on living in a "fantasy world", he says.
To encourage vigilance and suspicion will do far more harm than good, he argues. "It would lead to a society no-one wants to live in." Every so often a madman comes out of the woodwork and commits a horrific act. But there's no way of stopping that.
What kind of behaviour is suspicious is an "exceedingly grey area", Rudham admits. In the case of hearing the neighbours having a serious row in the garden, when should you intervene? "The point will come if things start getting thrown or actual violence starts. You wouldn't want to be reporting everyone having a row."
Yet, anyone campaigning on the issue of domestic violence will point out how often the offender will escalate to more and more serious crime.
No-one likes to think of a killer living in their midst, Wilson says. But picking up the signals might be almost impossible. Britain's most prolific serial killer was Harold Shipman, a trusted GP living in the community. "The probability with murder is that the perpetrator will be in some kind of relationship with the victim", he says.
"The murder rate is declining and the phenomenon of murder is an intrafamiliar one. Children below the age of two are the most common murder victim."
The best way to tackle this is to take domestic violence seriously. This means better training for social workers and encouraging police to take stronger action in domestic disputes.
There can be a problem with suspicions of domestic violence being reported, says Rudham. When his Neighbourhood Watch group in Leicestershire surveys local residents, 90% will say that domestic violence is one of their main concerns.
"But we can't pass that information to anyone. We don't know if they are victim or the perpetrator (or neighbour). The real serious point is they won't let you tell anyone."