An accident occurred on 19 March 1981 that led to the deaths of three people. During a countdown test for STS-1, a pure nitrogen atmosphere was introduced in the aft engine compartment of "Columbia" to reduce the danger of an explosion from the many other potentially dangerous gases on board the orbiter. At the conclusion of the test, pad workers were given clearance to return to work on the orbiter, even though the nitrogen had not yet been purged due to a recent procedural change. Three technicians, John Bjornstad, Forrest Cole, and Nick Mullon, entered the compartment without air packs, unaware of the danger since nitrogen gas is odorless and colorless, and lost consciousnesses due to lack of oxygen. Several minutes later, another worker saw them and tried to help, but passed out himself. The fourth did not alert anyone, but was himself seen by two other people. Of those two, one alerted a security guard and another went to help the unconscious group. The security guard entered the compartment with an air pack and removed the five men from the compartment.