Could steroids be a factor in Moat's alleged behaviour?
It is claimed that suspected gunman Raoul Moat, who is being hunted by police in connection with the shooting of his former girlfriend, her new partner and a police officer, has taken steroids.
This might explain the 6ft 3in tall ex-bouncer's muscular build. But can steroids - which have been linked to aggression - be blamed at all for Moat's alleged behaviour?
Family members said that he used body-building steroids and that he was prone to "eruptions of anger".
Some research suggests that both men and women can develop extremely aggressive behaviour when they use anabolic steroids, but other experts point to a lack of hard evidence.
A letter purporting to be from Raoul Moat, published by The Sun newspaper, said: "It's like the Hulk, it takes over and it's more than anger and it happens only when I'm hurt."
But it also said: "Obviously I have issues but I was pushed. I never beat my kids. I could simply admit to anything now cos it doesn't matter."
According to the charity DrugScope, many people who use anabolic steroids are 'self improvers'.
They like to think of themselves as healthy and fit and getting ahead in the world. Even if they are injecting, they don't think of themselves as drug-users, any more than somebody who regularly takes vitamin pills.
Like Moat, many have been doormen or security guards. But young men are increasingly getting involved in taking steroids to improve their body image, rather than for sport-related body-building, says the charity.
Anabolic steroids are drugs that are usually synthesised from the male hormone testosterone.
They have been banned by many sporting bodies, including the International Olympic Committee, because of their danger to health and the potential for competitive advantage.
They can cause serious damage to many of the body's major organs, particularly the liver, whose job it is to break down the compound.
There is also a risk of damage to the heart, which is made of muscle tissue. The cardiac muscle can expand, which can cause heart attacks.
Men taking steroids may also develop breasts because excessive levels of testosterone can be converted into the female sex hormone oestrogen.
On top of this there may be impotence, shrinking of the testicles, reduced sperm count, infertility, and increased risk of prostate cancer.
As well as the physical risks, steroids can also have an impact on mood and behaviour.
Research in athletes who use anabolic steroids suggests that both men and women can develop extremely aggressive behaviour.
Some athletes believe this can be advantageous, to promote harder and more intensive training.
But it can also be destructive.
Abuse can cause dramatic mood swings, depression, paranoid jealousy, extreme irritability, delusions and impaired judgement.
Chris Hudson, a nurse from drugs information service Frank, said: "The drugs can make normally calm people feel irritable, aggressive and even violent. They can cause dramatic mood swings and lead to paranoia, confusion and insomnia."
But there is very little hard and fast evidence linking steroid misuse to actual behaviour changes.
Jim McVeigh, an expert on the subject of substance abuse at the Centre for Public Health at Liverpool John Moores University said the evidence was "quite thin".
"To date it's not been possible to prove a causal effect between anabolic steroids and aggression.
"Part of the problem is that a lot of the side-effects are self-reported so there is a high level of bias.
"Also, because of the nature of the drugs used it is rare to identify the exact drug used. It's an illicit market - most of the drugs are fake or counterfeit and we do not know what's in them."
And Mr McVeigh said it would be impossible to say if any alleged history of steroid misuse would have any bearing on behaviour.
"Like any drug, some people will be fine taking it and others will experience side-effects.
"But you can't separate out the effects of the drug from other factors like lifestyle and personality traits when it comes to behaviour. Steroid use is just one of a number of factors to consider."