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The Russian Soyuz spacecraft responsible for last week's leak aboard the International Space Station (ISS) may have received its wounds here on Earth, on the grounds of its manufacturer, according to a new report from Russian news agency TASS.

ISS controllers noticed a slight pressure drop on the night of Aug. 29 and alerted crewmembers about it the next day. The astronauts traced the issue to a 2-millimeter (0.08 inches) hole in the upper "orbital module" of the crew-carrying Soyuz, which arrived at the station in June.

Cosmonaut Sergey Prokopyev, the Soyuz commander, soon patched the hole with epoxy, apparently solving the problem. Pressure levels have been steady ever since, NASA officials have said. (The leak never put crewmembers in any serious danger, officials have stressed.) [Russia's Crewed Soyuz Space Capsule Explained (Infographic)]

Space station astronauts patched a small hole in the upper orbital module of the Soyuz MS-09 spacecraft (left) on Aug. 30, 2018. Russian space officials have said that hole was likely caused by a drill here on Earth; investigators are currently trying to figure out exactly what happened.
Space station astronauts patched a small hole in the upper orbital module of the Soyuz MS-09 spacecraft (left) on Aug. 30, 2018. Russian space officials have said that hole was likely caused by a drill here on Earth; investigators are currently trying to figure out exactly what happened.
Credit: NASA/Space.com

The cause of the hole remains under investigation, however. Early speculation centered on a possible micrometeoroid strike, but now human error is strongly suspected. Indeed, the hole's circular shape suggests a drilling mishap, as do nearby marks on the module wall.

The incident may have occurred during the final assembly or testing of the Soyuz, according to the new report, published today (Sept. 6) by the Russian news agency TASS. Both of these activities take place at facilities run by the Soyuz's builder, Russian aerospace company Energia, in the city of Korolyov, near Moscow.

"One of the possibilities is the spacecraft might have been damaged in the final assembly hangar. Or it could happen at the control and testing station, which carried out the final workmanship tests before the spacecraft was sent to Baikonur," an unnamed source in the aerospace industry told TASS, which stressed that it has not confirmed such suspicions.

Soyuz spacecraft — which have been astronauts' only ride to and from the ISS since NASA's space shuttle program retired in 2011 — launch from Baikonur Cosmodrome, in Kazakhstan.

The Soyuz passed pressure-chamber tests before going to Energia's final-assembly hangar, the source told TASS. And the assembly and testing facilities are tightly controlled spaces, he added. (TASS referred to the source as a "he.")

"Only those with proper security clearance are allowed to enter," the source said. "Also, at the entrance to the hangar and the control and measurement station there are security guards checking all those who come and go."

Energia is conducting an investigation into the Soyuz incident. And Dmitry Rogozin, the head of Russia's federal space agency, Roscosmos, has vowed to find the person or persons responsible.

The orbital module is a spherical portion of the Soyuz that allows more gear to go up with the spacecraft. Unlike the lower crew capsule, the orbital module does not survive re-entry into Earth's atmosphere.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.