A Russian cosmonaut who explored a mysterious hole in a capsule docked to the International Space Station has revealed that the opening was drilled from inside the spacecraft.

It's been four months since crew first discovered the hole in the Russian Soyuz spacecraft attached to the orbiting lab, but just how it got there remains a mystery.

On Monday, Sergey Prokopyev said Russian law enforcement agencies are now examining samples he and crewmate Oleg Kononenko collected during a Dec. 12 spacewalk in effort to find out what caused it.

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The crew quickly located and sealed the tiny hole that created a slight loss of pressure. Space officials said the station has remained safe to operate. There appear to be drill marks around the hole on the inside (pictured). Despite ongoing investigation, no one knows how it got there

Prokopyev and two other astronauts returned to Earth last week from a 197-day space station mission.

The hole in the Russian Soyuz spacecraft attached to the station was spotted on Aug. 30.

Just days prior to their return to Earth, the cosmonauts endured a grueling spacewalk that lasted almost eight hours to investigate the hole, using knives and shears to carve into the side of the ISS.

The crew discovered a leak that was creating a slight loss of pressure and plugged the hole with epoxy and gauze.

Mr Prokopyev and Mr Kononenko had to use a pair of telescoping booms to reach the Soyuz and said it took nearly four hours for them to cross the approximately 100 feet to get to the capsule (pictured)

A tiny hole, only 2mm wide (pictured), was found to be responsible for the loss of valuable cabin pressure which was first detected on 30th August

Prokopyev said at a news conference the cavity started from the capsule's interior and 'it's up to the investigative organs to judge when that hole was made.'

The astronauts' quick identification and repair of the hole demonstrated 'the crew was ready for any developments,' he said.

The hole didn't pose a danger to Prokopyev and crewmates Serena Aunon-Chancellor of NASA and Alexander Gerst of the European Space Agency during their return because the section of the capsule it appeared on was jettisoned before the fiery re-entry into Earth's atmosphere.

Russian cosmonaut Sergey Prokopyev, crew member of the mission to the International Space Station, ISS, waves as he boards the spaceship prior to the launch at the Russian leased Baikonur cosmodrome, Kazakhstan

WHAT COULD HAVE CAUSED A HOLE IN THE ISS? Theory one - it was caused by a small meteorite A tiny hole appeared in a Russian space capsule locked to the ISS on 30th August. The 'micro fracture' believed to be around 2mm wide in the $150 billion (£115 billion) space station was discovered after astronauts noticed a drop in pressure. European Space Agency astronaut Alexander Gerst reportedly put his finger over the hole before crew patched it with tape. The hole was confirmed repaired by Friday (31 August) after cabin pressure returned to normal. It was initially believed to have been caused by a small meteorite and astronauts used tape to seal the leak after it caused a minor loss of pressure. Theory two - it was made deliberately while in orbit However, as the investigation went on it began to look like the hole was made from someone inside as opposed to outside, either back on Earth or in space, the Russian space agency claimed. Roscosmos chief Dmitry Rogozin said in September that the hole could have been drilled during manufacturing or while in orbit with a 'wavering hand'. He didn't say if he suspected any of the US crew, but the statement has caused some bewilderment. Sources suggest the question of how to fix the hole may have strained relations between Moscow and Houston. Rogozin has since reneged on his statement blaming the media for twisting his words and said that he 'never pointed the finger at U.S. astronauts'. Theory three - it was caused by a worker at Energia A leading theory from an unnamed source at Energia said the hole was made on the ground – potentially caused by 'deliberate interference' – with suggestions the person responsible may have already been identified. Another anonymous source said the hole was drilled by a worker who hid their mistake with a seal instead of reporting it. An unnamed source at Energia told the Russian news agency RIA Novosti that '[t]he hole was made on the ground'. According to the source, '[t]he person responsible for the act of negligence has been identified'. Another anonymous source said the hole was not made intentionally but by a worker who hid their mistake with a seal instead of reporting it. The patchwork repair lasted the trip up to the ISS but after three weeks in orbit gradually peeled away. Advertisement

Roscosmos chief Dmitry Rogozin said in September the hole could have been drilled when the capsule was manufactured or in orbit.

Rogozin stopped short of blaming crew members, but the statement has caused some friction between Roscosmos and NASA.

Rogozin has since back-pedaled from the statement, blaming the news media for twisting his words.

Prokopyev scoffed at the idea the hole could have been drilled by an astronaut, saying, 'You shouldn't think so badly of our crew.'