The fault was traced to a breathing ventilation valve, located between the orbital module and the descent module, that had been jolted open as the descent module separated from the service module, 12m 3s after retrofire. The two were held together by explosive bolts designed to fire sequentially; in fact, they fired simultaneously. The force of this caused the internal mechanism of the pressure equalization valve to loosen a seal that was usually discarded later and normally allowed automatic adjustment of the cabin pressure. The valve opened at an altitude of 168 kilometres (104 mi), and the resultant loss of pressure was fatal within seconds. The valve was located beneath the seats and was impossible to find and block before the air was lost. Flight recorder data from the single cosmonaut outfitted with biomedical sensors showed cardiac arrest occurred within 40 seconds of pressure loss. By 15m 35s after the retrofire, the cabin pressure was zero, and remained there until the capsule entered the Earth's atmosphere. Patsayev's body was found positioned near the valve, and he may have been attempting to close or block the valve at the time he lost consciousness.

Columbia's crew died below the Karman Line, all other deaths in the various space programs were well within the atmosphere.