Rumours are circulating that SpaceX is investigating sabotage as the cause of its Falcon 9 explosion at the start of September.

While reviewing footage of the unexplained explosion, unnamed industry officials report that SpaceX saw an "odd shadow and then a white spot" on the roof of a nearby building just before the blast - a building that's owned by long-standing competitor United Launch Alliance (ULA).

The footage is being kept under wraps for now, but Christian Davenport from The Washington Post reports that a SpaceX employee tried to enter the ULA building to investigate after seeing the video, but was denied access. ULA says US Air Force investigators have since checked out the roof and found nothing suspicious.

So far, SpaceX hasn't gone on the record about a sabotage investigation, so we don't have a lot of evidence to go on, but last week, Elon Musk told a conference in Mexico that the source of the explosion is still unknown, and has also called it "the most difficult and complex failure we have ever had in 14 years".

"We’ve eliminated all of the obvious possibilities for what occurred there," Musk told the International Astronautical Congress last week. "So what remains are the less probable answers."

The mystery stems from the fact that the explosion occurred during a routine tank filling - the engines weren't on, and SpaceX hasn't been able to identify a heat source for the explosion.

Musk has publicly called on NASA, the Air Force, and the US Federal Aviation Administration (FAA) to help with the investigation.

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

That doesn't mean in any way that sabotage or ULA was involved - right now all we have to go on are rumours and speculation, with no one going on the record about anything.

All we know so far is that the ULA rooftop where the suspicious activity was reportedly spotted is just over a mile away from SpaceX's launchpad, and has a clear line of sight to it, according to The Washington Post.

Adding to the intrigue is the fact that ULA, which is co-owned by Lockheed Martin and Boeing, has a history of competition with SpaceX.

Up until 2014, ULA had a monopoly on US government security contracts worth hundreds of millions of dollars, and was the only launch provider certified by the Air Force.

But two years ago, SpaceX sued the Air Force for the right to compete and won - enabling it to perform supply runs for the International Space Station, among other important satellite delivery jobs.

Now, in the aftermath of the Falcon 9 explosion, ULA chief executive Tory Bruno - who was hired specifically to compete with SpaceX - has written a letter to Pentagon officials asking them to reconsider using the more affordable SpaceX, based on safety and reliability concerns. Ouch.

To add insult to injury, last week, "10 Republican House members, many friendly to ULA, told NASA that SpaceX should not be leading the investigation and that authority should be turned over to the federal government," reports The Washington Post.

There's been no comment from the Air Force or ULA since then, since the investigation is ongoing.

Again, we have no solid evidence as yet that ULA was in any way involved in the SpaceX explosion - or that sabotage played a part at all - so put those pitchforks away. Only time will tell what officials might find, but we'll be watching closely.