On September 1, one of SpaceX’s Falcon 9 rockets exploded during pre-launch prep, and now Elon Musk has taken to Twitter to seek clues about the cause from any third-party footage of the incident that may exist. Plus, he’s not ruling out some kind of third-party interference.

In a series of tweets early Friday, Musk noted that SpaceX is “still working” on the investigation of the incident, and that they’ve already found that the explosion is “turning out to be the most difficult and complex failure we have ever had in 14 years.”

Musk did add some specifics from their findings, too, noting that the engines were not active and therefore, there shouldn’t have been any heat source to trigger the blast.

He also said that the investigation is particularly curious about a first, “quieter bang sound” which happens just a bit before the fireball and larger noise from the explosion. Musk noted this “may come from rocket or something else,” and in response to a question on Twitter, also said that they haven’t ruled out the possibility that, as at least a few YouTube videos and one Reddit user who claims to have been working at the site have suggested, an external object of some kind struck the rocket and led to the explosion.

We have not ruled that out. — Elon Musk (@elonmusk) September 9, 2016

Musk is seeking any additional footage of the explosion, asking via Twitter the people send it to report@spacex.com. The call for external video almost certainly means data on the ground isn’t providing what they need in terms of information to come up with an answer, and different angles on what was going on outside the rocket could provide a key ingredient in arriving at an answer that can help SpaceX move forward.