Shortly after the Falcon 9 rocket explosion on June 28, SpaceX CEO Elon Musk tweeted a possible cause of the accident. But nine days and thousands of research hours later Musk has admitted that the reason for the explosion remains a mystery.

"The data does seem to be quite difficult to interpret," Musk said at an International Space Station R&D conference in Boston on July 7. "Whatever happened was not straightforward."

Musk confirmed what he mentioned in his June 28 tweet that there was an overpressure event in the upper stage liquid oxygen tank, but admitted that in terms of "the exact cause and the sequence of events, there is still no clear theory that fits with all the data."

There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause.

— Elon Musk (@elonmusk) June 28, 2015

  "It's a huge blow to SpaceX. We take these missions incredibly seriously," Musk told the assembled crowd at the webcast session at the ISS R&D Conference. Failure to find the cause of the accident would be a disaster as the next launch is currently postponed until a clear reason can be found for the explosion. Musk didn't reveal any new details about the explosion, saying he was wary of misleading the large media presence in the Boston audience, but said he expected to be able to release more details by the end of the week. The latest public information comes from when Musk tweeted the day after the accident that his team was still trying to find out what happened in of the final milliseconds before the explosion, but he didn't elaborate on this investigation.

Cause still unknown after several thousand engineering-hours of review. Now parsing data with a hex editor to recover final milliseconds. — Elon Musk (@elonmusk) June 29, 2015

The Falcon 9, which was carrying supplies for the International Space Station, was the third cargo mission to fail in the last 8 months. Another private U.S. company, Orbital ATK, has halted operations since a launch accident in October. An April Russian cargo mission ended when the spacecraft burned up in the Earth's atmosphere, although one much-needed Russian supply ship did eventually reach the ISS on July 5.

Orbital ATK is aiming to restart missions in March 2016 and since the Falcon 9 explosion NASA has confirmed its support for SpaceX in future cargo and manned space missions. Musk confirmed that SpaceX is still on course to bring NASA astronauts into space in 2017. "Things seem to be going fairly well in the commercial crew front," Musk said. "Overall, there are small disagreements here and there, but overall we very much agree with the way it's being done. It's pretty good."

But for now the main focus will be to find out exactly what happened to the Falcon 9 so that it can continue its ambitious schedule of launches as soon as possible.