Over a couple tweets from Friday morning, Musk noted that the company was "still working" on the investigation which was "turning out to be the most difficult and complex failure we have ever had in 14 years." The biggest mystery, apparently, is how the explosion occurred during a routine filling operation, when there was "no apparent heat source." According to Musk, SpaceX is trying to get to the bottom of a bang sound that can be heard a few seconds before the explosion and the company is seeking any third-party recordings of the event so they can get a better view, or at least a different angle, on the incident.
As TechCrunch notes, the call for outside sources indicates SpaceX's own data must not be very helpful in this case. The company is also exploring one theory from a Reddit user who was allegedly at the site that suggests the explosion was caused by another object striking the rocket.