SpaceX is zeroing in on what caused the massive explosion that ripped a rocket to pieces on a launch pad in Florida two months ago, and the discovery is critical if the company wants to move forward with future missions for NASA. SpaceX CEO Elon Musk has called it the “toughest puzzle” they’ve ever had to deal with, and it also may be the most important.

A Falcon 9 rocket exploded on the launch pad back on Sept. 1, and for weeks scientists couldn’t figure out why, which proved to be a major problem for a company that hopes to be launching a large quantity of flights for NASA. But Musk says SpaceX now believes it was the liquid helium, advanced carbon fiber composites and solid oxygen that appeared to be behind the explosion, and the discovery hopefully enables the company to get back to launching spacecraft as early as December.

The problem appears to be the supercooled liquid oxygen used by SpaceX as propellant became so cold it turned into a solid, and may have reacted with a piece of hardware, such as the liquid helium pressure vessels.

“The Accident Investigation Team continues to make progress in examining the anomaly on September 1 that led to the loss of a Falcon 9 and its payload at Launch Complex 40 (LC-40), Cape Canaveral Air Force Station, Florida,” SpaceX said in a statement. “Since the incident, investigators from SpaceX, the FAA, NASA, the US Air Force and industry experts have been working methodically through an extensive fault tree to investigate all plausible causes. As part of this, we have conducted tests at our facility in McGregor, Texas, attempting to replicate as closely as possible the conditions that may have led to the mishap.

“The investigation team has made significant progress on the fault tree,” it continues. “Previously, we announced the investigation was focusing on a breach in the cryogenic helium system of the second stage liquid oxygen tank. The root cause of the breach has not yet been confirmed, but attention has continued to narrow to one of the three composite overwrapped pressure vessels (COPVs) inside the LOX tank. Through extensive testing in Texas, SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions. These conditions are mainly affected by the temperature and pressure of the helium being loaded.”