Knots and Ligatures

Ligatures are materials such as ropes or wire that are used to tie or bind; they have various roles in criminal acts. Knots and ligatures may be used are used to bind, restrain, strangle, or hang victims. Their analysis is a specialized branch of forensic science .

A ligature is an important form of physical evidence . Ligatures can be made from rope, electric flex, nylon, clothing, bedsheets, chains, dog leads, washing lines, luggage straps, or various other objects. The perpetrator may come prepared, armed with rope perhaps, or may use what is at hand. However, an assailant may carry traces of the ligature material away, and this can be used, if his clothing is examined, to link him with the scene of the crime. Ligature materials are class evidence , able to link a suspect to a type of material, rather than individualizing evidence, which can link a suspect to a particular portion of material.

A ligature is generally used by making a knot within the material. There are several different types of knot that can be identified by the forensic expert, such as slipknots, reef knots, and overhand knots. This may be revealing of certain characteristics of the person who tied it, such as knot-tying skill, trade, and hobbies.

Like the ligature from which it is made, the knot is an important item of physical evidence. When found at the scene of the crime, it must first be carefully photographed. When handled, the utmost care must be taken not to actually untie it. If a victim has been tied, then it will have to be removed, but will be photographed as this is happening. When it is cut away, the ligature is severed in a direction away from the knot so as better to preserve it. Great care has to be taken in handling loose knots so they do not disintegrate. Knots made in wire may be obscured by dirt or mud, but their intricacies can be revealed by fluoroscopy or x-ray analysis. The knot itself may bear important evidence such as fingerprints or hairs and this should be collected before the knot itself is removed.

Knot and ligature analysis may be especially important in the investigation of cases of strangulation, which are often homicide. The ligature is gradually tightened around the neck until compression on the airways and blood vessels in the neck produces asphyxia, where oxygen supplies to the body and brain are cut off. The mark on the neck in such cases tends to reflect the material used for the ligature. If wire or thin cord was used, there will be a clear-cut, deep mark with sharply defined edges. If a soft fabric is pulled tight around the neck, it will tend to fold up into a series of ridges that will show up as interlacing areas of bruising. Sometimes the mark reproduces the pattern of the ligature, such as a weave mark whose width may suggest its size. Sometimes the ligature is left on the neck and, if so, the position of any knots gives the investigator a good idea of where the attacker was relative to the victim.

see also Asphyxiation (signs of); Hanging (signs of).