[citation needed] The Soyuz 7K-OKS spacecraft was launched on 6 June 1971, from the Baikonur Cosmodrome in the central Kazakh Soviet Socialist Republic, and used the callsign Yantar (Amber).

Docking took 3 hours 19 minutes to complete and involved making the connection mechanically rigid, engaging various electrical and hydraulic links, and establishing air-tight seals before locks could be opened.

[9] Upon first entering the station, the crew encountered a smoky and burnt atmosphere, and after replacing part of the ventilation system, spent the next day back in their Soyuz until the air cleared.

Almost 25 minutes later, Soyuz 11's automatic systems landed the craft at 23:16:52 GMT, 90 kilometres (56 mi; 49 nmi) southwest of Karazhal in Kazakhstan, after an abnormally silent return to Earth.

On opening the hatch, they found all three men in their couches, motionless, with dark-blue patches on their faces and trails of blood from their noses and ears.

Flight recorder data from the single cosmonaut outfitted with biomedical sensors showed cardiac arrest occurred within 40 seconds of pressure loss.

An extensive investigation was conducted to study all components and systems of Soyuz 11 that could have caused the accident, although doctors quickly concluded that the cosmonauts had died of asphyxiation.

[18] The autopsies at Burdenko Main Military Clinical Hospital found that the cause of death for the cosmonauts was haemorrhaging of the blood vessels in their brains, with lesser amounts of bleeding under their skin, in their inner ears, and in their nasal cavities, all of which occurred as exposure to a vacuum environment caused the oxygen and nitrogen in their bloodstreams to bubble and rupture vessels.

After the flight, Leonov went back and tried closing one of the valves himself, and found that it took nearly a minute to do so, too long in an emergency situation with the spacecraft's atmosphere escaping fast.

[17] The Soviet state media attempted to downplay the tragic end of the mission, and instead emphasized its accomplishments during the crew's stay aboard Salyut 1.

Since they did not publicly announce the exact cause of the cosmonauts' deaths for almost two years afterwards, United States space planners were extremely worried about the upcoming Skylab program, as they could not be certain whether prolonged time in a micro-g environment had turned out to be fatal.

[6] The cosmonauts were given a large state funeral and buried in the Kremlin Wall Necropolis at Red Square, Moscow, near the remains of Yuri Gagarin.

[17] The United States sent Tom Stafford, then NASA's Chief Astronaut, to represent President Richard Nixon at the funeral, where the Soviets asked him to be one of the pallbearers.

It will, I am sure, prove to have contributed greatly to the further achievements of the Soviet program for the exploration of space and thus to the widening of man's horizons.The Soyuz spacecraft was extensively redesigned after this incident to carry only two cosmonauts.

A memorial monument in the form of a three-sided metallic column, with the engraved image of the face of each crew member set into a stylized triangle on each of the three sides, was placed at the site.

[23] In 2013, Russian space agency Roscosmos restored the site with a redesigned monument, reflecting the three-sided form of the original, but this time constructed from brick.

39, in honour of the dead cosmonauts, a memorial stele was made with quotes from the poem by the poet Yevgeny Yevtushenko "Between our Motherland and you is a two-way eternal connection" (Russian: "Между Родиной нашей и вами – двусторонняя вечная связь").