Earlier this month, a Falcon 9 rocket built by SpaceX exploded on the launchpad, causing a blast that reportedly shook buildings several miles away. SpaceX is now scrambling to find out what went wrong, and this morning, CEO Elon Musk even tweeted a request for footage of the event from any witnesses.

Musk's tweets suggest that the exact details of what went wrong are unknown. "Still working on the Falcon fireball investigation," writes Musk. "Turning out to be the most difficult and complex failure we have ever had in 14 years." SpaceX originally said the explosion had been caused by an "anomaly on the pad," and later clarified that the problem occurred during the loading of propellant onto the rocket.

Here's what Musk has said this morning:

Still working on the Falcon fireball investigation. Turning out to be the most difficult and complex failure we have ever had in 14 years. — Elon Musk (@elonmusk) September 9, 2016

Important to note that this happened during a routine filling operation. Engines were not on and there was no apparent heat source. — Elon Musk (@elonmusk) September 9, 2016

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

You can listen out for the "quieter bang" (and the much larger one) in the video of the explosion below, but replying to fans on Twitter, Musk noted that SpaceX hadn't ruled out the possibility that something hit the rocket. Musk noted, though, that aliens weren't the cause.

Musk also tweeted that he would continue working on a blog post detailing updates to Tesla's Autopilot software tomorrow — a post he started writing and promised to publish soon before the explosion took place. This delay, though, will be the least of Musk's worries, as SpaceX's scheduled launches are put on hold as the investigation continues.

Tory Bruno, president and CEO of rival aerospace firm United Launch Alliance (ULA), told Reuters yesterday that it could take as long as a year for SpaceX to return to regular launches. "It typically takes nine to 12 months for people to return to flight," said Burno. "That's what the history is."

SpaceX made history with the Falcon 9 in April