A serial killer's crimes are nearly always alike in some way. The killer may only attack a certain type of person, for instance women of a certain age or appearance (young or old, blonde or dark-haired). He may always use the same kind of weapon to kill his victims, or may treat them in a certain manner, such as tying them up with the same kind of rope or adhesive tape. Some serial killers have had very strange habits which leave evidence of their work at each crime; for instance Albert de Salvo, the Boston Strangler, would tie the cord with which he had strangled a woman around her neck in a special bow. To law enforcement officers a serial killer's habits are known as his signature, and they are always valuable leads in detecting who he is and capturing him.