I've looked at other responses on Buddhism exchange and googled it too, and I'm still confused about the cause of suffering. Is there one root cause? Multiple causes? Or does Buddhism claim not to know the root cause but only causes later in the chain of causation?
Britannica seems to equate craving and attachment and suggest that is what Buddha considered the cause:
"The second truth is the origin (Pali and Sanskrit: samudaya) or cause of suffering, which the Buddha associated with craving or attachment in his first sermon. In other Buddhist texts the causes of suffering are understood as stemming from negative actions (e.g., killing, stealing, and lying) and the negative mental states that motivate negative actions (e.g., desire, hatred, and ignorance)."
My initial understanding had been that it was not desire that caused suffering (we all desire things, like if we're cold we desire warmth, and even experienced meditators will feel that way) but becoming attached to a desire or identifying with it. But now I don't know anymore.