In 1974, 132 officers were killed in handgun assaults in the United States. To prevent these senseless tragedies from reoccurring, research was conducted on ways to increase police body protection. Kevlar, first developed and used in police car tires, is now extensively used in police protective garments to stop bullet penetration. The Edgewood Arsenal (part of the U.S. Army) in Aberdeen, Md., conducted tests on Kevlar for both comfort and ballistic protection. The tests showed that seven layers of Kevlar 29 were both comfortable and adequate for everyday use, but that for protection from very powerful weapons such as a .357 magnum, more layers of the material were required. The arsenal also conducted tests on the effects of blunt trauma, caused by high-energy rounds, in which the soft body tissue is distorted even when the bullet does not penetrate the vest. Officers are told that protection is based on three factors: the number of layers of Kevlar and the quality and weave of the fabric. They are cautioned that none of the soft body armor garments will protect against rifle fire, although they will protect against knife wounds. The comfort of the garment is based on its design. Most vests weigh under 4 pounds.