SpaceX said today that it continues to narrow the investigation into what caused the on-pad fire that destroyed a Falcon 9 rocket and its Amos-6 satellite on September 1. The root cause still has not been determined, although the company hopes it can resume launches before the end of this year.

The Falcon 9 rocket was engulfed in flames and exploded during a standard pre-launch test two days before the scheduled launch of the Israeli-built Amos-6 communications satellite, which also was destroyed. The launch pad, Launch Complex 40 (LC-40), at the Air Force’s Cape Canaveral Air Force Station (CCAFS), was damaged. SpaceX leases the pad from the Air Force.

SpaceX founder Elon Musk calls it the “most difficult and complex failure we have seen.” Earlier, the company said it was focusing on a breach in the cryogenic helium system of the liquid oxygen (LOX) tank in the rocket’s second stage. A video of the incident shows the fire beginning near that location. Today’s statement explains that engineers have narrowed the cause to one of three composite overwrapped pressure vessels (COPVs) inside that LOX tank. They have been able to recreate a COPV failure “entirely through helium loading conditions” that are “mainly affected by the temperature and pressure of the helium being loaded.”

They are now focusing on finding the exact root cause and “developing improved helium loading conditions,” implying that they consider this a procedural rather than hardware-related issue.

The statement asserts that they are still working towards a resumption of Falcon 9 launches “before the end of the year” from launch pads at NASA’s Kennedy Space Center (KSC) in Florida or Vandenberg Air Force Base in California. It does not mention the status of LC-40.

SpaceX has several launch site options. In addition to LC-40, it leases Launch Complex 39A (LC-39A) from NASA at Kennedy Space Center (KSC), adjacent to CCAFS, and Space Launch Complex 4 (SLC-4) at Vandenberg Air Force Base (VAFB), CA from the Air Force (it has two pads, 4E and 4W). VAFB is used for launches into polar orbits that circle the Earth’s poles, as opposed to lower inclination orbits that benefit from launching in an easterly direction from Florida.

The announcement said both of those launch sites “remain on track to be operational in this timeframe.” SpaceX has already launched Falcon 9s from SLC-4E at VAFB. Its first launch from KSC’s LC-39A was expected this year, but of a different version of Falcon, the first flight of Falcon Heavy. It is not clear now when that rocket will make its debut.

SpaceX also is building its own launch site near Brownsville, Texas, although it has said little about that the status of that site in recent months.