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Revue Bulgare D'histoire. Publishing House of the Bulgarian Academy of Sciences. 1986. p. 68. Archived from the original on 2020-09-26. Retrieved 2019-06-30.
Gwertzman, Bernard (July 2, 1971). "Russians Mourn Soyuz 11 Astronauts". The New York Times. p. 14; Teitel, Amy Shira (August 20, 2017). "Only Three People Have Died in Space". Discover Magazine. Archived from the original on July 2, 2020. Retrieved June 30, 2020.