Another factor forensic investigators look at in a dead body is rigor mortis. This Latin term describes the state of a dead body. A few hours after a person dies (depending on conditions), the joints of a dead body stiffen and become locked in Rigor mortis is caused by chemical changes in the body after death due to place. Oxygen depletion and the buildup of calcium. When a person is alive, the calcium in a body is carried outside the body's cells. This allows muscle fibers to move But since a dead body is no longer receiving oxygen, calcium builds up causing the muscles to stiffen up and contract.