When a SpaceX rocket exploded on the launch pad in early September, the initial speculation about the disaster was that it was a simple accident. Building rockets and going to space is incredibly hard, especially for a private company, and this wasn’t the first time SpaceX had failed on their mission to make space travel cheaper and easier. 

But on Friday, the Washington Post reported that investigators were considering a different explanation: sabotage, possibly by United Launch Alliance, a fierce rival of SpaceX.

On video taken the day of explosion, the Post reported, there appeared to be an “odd shadow, then a white spot on the roof of a nearby building leased by ULA.”

And, later, when a SpaceX employee asked to get access to the roof, presumably to investigate further, ULA denied SpaceX’s request. Access to the roof was later given to Air Force investigators, who didn’t find any evidence of sabotage. 

Still, the episode hints at something Elon Musk, SpaceX’s founder, has also gestured at, as the enigma of what happened that day in Cape Canaveral, Florida lingers. 

“We’ve eliminated all of the obvious possibilities for what occurred there,” Musk said recently, according to the Post. “So what remains are the less probable answers.”

Sabotage or not, the rocket’s explosion has confounded investigators. As Musk has written on Twitter, it occurred during a routine operation, and there were no apparent heat sources nearby. All investigators know now is that it was set off by a breach in the rocket’s helium system. How that breach occurred remains a mystery. 

“Still working on the Falcon fireball investigation,” Musk has said. “Turning out to be the most difficult and complex failure we have ever had in 14 years.”