After an investigation, it was determined that a valve which was not supposed to open until an altitude of 4 km was reached had been jerked open during the undocking maneuver, causing the cosmonauts oxygen to bleed out into space. The crew tried to crank the valve closed, but did not have time. Due to space limitations, they were not wearing space suits. Official Soviet documentation says:
"At approximately 723 seconds after retrofire, the 12 Soyuz pyro cartridges fired simultaneously instead of sequentially to separate the two modules .... the force of the discharge caused the internal mechanism of the pressure equalization valve to release a seal that was usually discarded pyrotechnically much later to adjust the cabin pressure automatically. When the valve opened at a height of 168 kilometers the gradual but steady loss of pressure was fatal to the crew within about 30 seconds. By 935 seconds after retrofire, the cabin pressure had dropped to zero....... ...only trough analysis of telemetry records of the attitude control system thruster firings that ahd been made to counteract the force of the escaping gases and through the pyrotechnic powder traces found in the throat of the pressure equalization valve were Soviet specialists able to determine that the valve had malfunctioned and had been the sole cause of the deaths."
The USSR did not attempt to send any other crews to Salyut 1, which was later deorbited and allowed to burn up on reentry. It was more than two years before another manned mission was attempted. Later crews were limited to two men, to allow room for the required space suits during take-off and landing.