A ground-based test of the Super Heavy rocket that is intended to boost a Starship upper stage into orbit ended in flames on Monday afternoon at SpaceX’s launch site in South Texas. A fire burned in the vicinity of the pad, on and off, for more than an hour.

This is the first time SpaceX has tested a booster stage—this one bears the designation Booster 7—equipped with a full complement of 33 Raptor rocket engines. Monday’s test was not intended to lead to a static fire test, during which the engines are briefly ignited, so seeing fire erupt from the aft end of the vehicle at 4:20 pm CT local time was a surprise.

The methane-fueled Raptor engine has a complicated sequence of events that must unfold precisely in order for it to ignite, and SpaceX was testing the “spin start” portion of this ignition sequence when the anomaly occurred. Something must have caused methane propellant to ignite, with the ambient oxygen in the air serving as an oxidizer, inside the vehicle.