On September 1, the unmanned SpaceX Falcon 9 rocket exploded during a routine pre-launch at Space Launch Complex 41 in Cape Canaveral, FL. The explosion, which took place two days before the scheduled launch, came as a great surprise to SpaceX, founded by billionaire Elon Musk. Musk, who is one of the founders of Tesla Motors and PayPal, tweeted updates following the accident and disclosed that the source of the explosion was still unclear.

Loss of Falcon vehicle today during propellant fill operation. Originated around upper stage oxygen tank. Cause still unknown. More soon.

— Elon Musk (@elonmusk) September 1, 2016

Scientists at SpaceX and NASA have been fervently investigating the cause of the explosion, which was especially confusing because the rocket’s engines were off and there was no heat source. SpaceX has previously launched many successful spacecrafts, such as the 2010 Dragon capsule and the 2012 Dragon CRS-1, the first SpaceX resupply launch to the International Space Station.

In a recording of the launch, SpaceX engineers noticed a subdued noise just before the explosion, which Musk tweeted was under investigation in particular. Some Twitter users responded with proposed explanations, such as Paul Marks, who suggested the noise could have been a failing battery cell.

Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off. May come from rocket or something else.

— Elon Musk (@elonmusk) September 9, 2016

Biology teacher, Paul Sloshberg, revealed his belief that the explosion was only a minor setback for SpaceX, saying, “Luckily, they [SpaceX] only lost some equipment, which they can recover from.”

Junior, Gabby Hogan, quickly added in, “Yeah, some expensive equipment!”

On September 9, the SpaceX Twitter account requested any photos or videos of the explosion and Musk tweeted at NASA asking for support and advice. There have been no updates so far, but progress in the investigation can be expected on SpaceX or Musk’s Twitter accounts.