A Russian rocket that carried two people to space last month failed and sent the craft back to Earth because of "deformation" of a part that was made during assembly at the cosmodrome, a space official said Thursday.

He said this caused a booster rocket from the first stage to malfunction and hit a fuel tank which "led to the loss of stabilisation" and triggered an emergency landing.

The Russian-American crew of two had to withstand a ballistic descent back to Kazakhstan on October 11, but both emerged from their landing craft safe and sound.

Executive director of Russia's Roscosmos space agency Sergei Krikalyov said Wednesday that the root of the problem was a sensor that indicated the separation of the first two stages of the Soyuz rocket.

Skorobogatov, who heads TsNIIMash, a Russian research institute specialising in spacecraft and missile development, said the commission ruled out that the problem happened at a production facility.

Russia is the only country currently able to send astronauts to the International Space Station, and the accident caused it to suspend all launches until getting to the bottom of the rare failed manned launch.

However the safe descent to Earth by cosmonaut Aleksey Ovchinin and US astronaut Nick Hague led both Roscosmos and NASA to stand by the Soyuz system as reliable.

The Soyuz "remains the most reliable rocket," said Dmitry Baranov, the acting director of Russia's Energia rocket and space corporation.

Following the investigation by the space experts, "appropriate law enforcement authorities" will work out who is guilty of the assembly mistake, said Roscosmos deputy head Alexander Lopatin.

"Every accident has a name and surname (of the guilty party)," he said.

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