SpaceX CEO Elon Musk says that his company has finally gotten to the bottom of the September 1st Falcon 9 explosion — claiming it was the “toughest puzzle” they’ve ever had to solve. And now that the problem is known, he expects SpaceX to return to flight in mid-December.

Speaking on CNBC yesterday, Musk said “it basically involves liquid helium, advanced carbon fiber composites, and solid oxygen. Oxygen so cold that it actually enters solid phase.”

“Oxygen so cold that it actually enters solid phase.”

So what does that mean exactly? Musk gave some hints a little while ago during a speech he gave to the National Reconnaissance Office. According to a transcript received by Space News, he argued that the supercooled liquid oxygen that SpaceX uses as propellant actually became so cold that it turned into a solid. And that’s not supposed to happen.

This solid oxygen may have had a bad reaction with another piece of hardware — one of the vehicle’s liquid helium pressure vessels. Three of these vessels sit inside the upper oxygen tank that holds the supercooled liquid oxygen propellant. They’re responsible for filling and pressurizing the empty space that’s left when the propellant leaves the tank. The vessels are also over wrapped with a carbon fiber composite material. The solid oxygen that formed could have ignited with the carbon, causing the explosion that destroyed the rocket.

SpaceX isn’t giving too many more details about the process, and the company declined to give further clarification about what Musk said on CNBC. Plus, it’s unclear what caused the solid oxygen to form. There’s speculation from the New York Times that if liquid helium was used in the pressure vessels, which Musk seems to have indicated, it might have been cold enough to freeze the liquid oxygen into a solid. Liquid helium exists at -452 degrees Fahrenheit, a lot colder than SpaceX’s liquid oxygen propellant at -340 degrees Fahrenheit. And oxygen solidifies at -361 degrees Fahrenheit.

Despite all this, SpaceX is confident about getting back to flight by the end of the year, based on what the company has found. And in a recent update, SpaceX claims to be focused on improving its helium loading processes so this accident doesn’t happen again.