"The only place where it could happen was during rocket assembly at the Baikonur cosmodrome", he said.
It was that explosion that caused the briefly-terrifying moment in the live launch video when the crew were briefly shaken around, prior to the capsule returning to land in Kazakhstan.
Although the official report on the cause of a Soyuz rocket failure won't be released until Thursday, a Russian official disclosed its central conclusion a day early, the country's news agency TASS reports.
Russian Federation said today the launch of a Soyuz rocket failed last month due to a sensor that was damaged during assembly but insisted that the spacecraft remains reliable.
A failed Russian Soyuz rocket launch three weeks ago that aborted after just two minutes was caused by a sensor that was damaged during assembly, according to an official investigation.
The accident was the first involving a manned launch since the Soviet era and delivered another blow to the image of Russia's space programme after embarrassing losses of satellites and of an unmanned cargo ship in 2015.
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Krikalyov blamed a "malfunction" of the sensor separating the first and second stages of the rocket for the problem and said that efforts were being taken to ensure the safety of future flights.
"The reason for the abnormal separation. was due to a deformation of the stem of the contact separation sensor.", Skorobogatov told reporters.
The other was now in Kourou, a space port in French Guiana which Russian Federation uses for commercial launches of satellites, he added.
Two more Soyuz rockets at the Baikonur and Kourou spaceports with the same defect have been discovered, Skorobogatov said, with additional checks introduced into the rocket assembly process.
When the Soyuz launch failed earlier this month, uncertainty over the ISS crew launch schedule led to speculation that the space station would have to be abandoned, at least for the time being.
On the rocket destined for the ISS will be Russian cosmonaut Oleg Kononenko, Canadian astronaut David Saint-Jacques, and NASA's Anne McClain.
"Every accident has a name and surname (of the guilty party)", he said.